### Summary

With the ability to create private automation hub servers for both community and AAP users, customers may end up with multiple galaxy servers in their ansible.cfg, AWX or Tower configuration. This could form part of a HA pattern where multiple prviate automation hub servers are deployed. When ansible-galaxy is given a list of galaxy servers I would expect ansible-galaxy to traverse the list of galaxy servers to install a collection. If a galaxy server in the list is down it should move to the next server in the list to try to install a collection rather than failing. 

### Issue Type

Bug Report

### Component Name

ansible-galaxy

### Ansible Version

```console
$ ansible --version
ansible [core 2.11.3] 
  config file = None
  configured module search path = ['/home/pharriso/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/pharriso/ansible-latest/lib64/python3.6/site-packages/ansible
  ansible collection location = /home/pharriso/.ansible/collections:/usr/share/ansible/collections
  executable location = /home/pharriso/ansible-latest/bin/ansible
  python version = 3.6.8 (default, Mar 18 2021, 08:58:41) [GCC 8.4.1 20200928 (Red Hat 8.4.1-1)]
  jinja version = 3.0.1
  libyaml = True
```


### Configuration

```console
$ ansible-config dump --only-changed
GALAXY_IGNORE_CERTS(/home/pharriso/ha-hub/ansible.cfg) = True
GALAXY_SERVER_LIST(/home/pharriso/ha-hub/ansible.cfg) = ['hub-2_rh-certified_repo', 'hub-1_rh-certified_repo']
```


### OS / Environment

Testing on RHEL8 but assume all platforms

### Steps to Reproduce

Create ansible.cfg with multiple galaxy servers defined.

```[galaxy]
server_list = hub-2_rh-certified_repo,hub-1_rh-certified_repo
ignore_certs = true

[galaxy_server.hub-2_rh-certified_repo]
url=https://hub-2.demolab.local/api/galaxy/content/rh-certified/
token=9c637928da99a2711359dc4ed5e5fd53166464e0

[galaxy_server.hub-1_rh-certified_repo]
url=https://hub-1.demolab.local/api/galaxy/content/rh-certified/
token=0b86a3cf21053bc6757a4e7a1c85951a124f9389
```

Power off one of the hub servers and then attempt to install a collection. Even if the hub server that is down isn't the first in the list the install still fails:

```bash
ansible-galaxy collection install phoenixnap.bmc --vv --force
```






### Expected Results

I would expect ansible-galaxy to ignore the galaxy server that is down and to continue through the list to see if another galaxy server can fulfill the request and allow the requested collection to be installed

### Actual Results

```console
ansible-galaxy [core 2.11.3] 
  config file = /home/pharriso/ha-hub/ansible.cfg
  configured module search path = ['/home/pharriso/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/pharriso/ansible-latest/lib64/python3.6/site-packages/ansible
  ansible collection location = /home/pharriso/.ansible/collections:/usr/share/ansible/collections
  executable location = /home/pharriso/ansible-latest/bin/ansible-galaxy
  python version = 3.6.8 (default, Mar 18 2021, 08:58:41) [GCC 8.4.1 20200928 (Red Hat 8.4.1-1)]
  jinja version = 3.0.1
  libyaml = True
Using /home/pharriso/ha-hub/ansible.cfg as config file
Starting galaxy collection install process
Found installed collection awx.awx:19.0.0 at '/home/pharriso/.ansible/collections/ansible_collections/awx/awx'
Found installed collection community.general:2.4.0 at '/home/pharriso/.ansible/collections/ansible_collections/community/general'
Found installed collection community.kubernetes:1.2.1 at '/home/pharriso/.ansible/collections/ansible_collections/community/kubernetes'
Found installed collection community.vmware:1.10.0 at '/home/pharriso/.ansible/collections/ansible_collections/community/vmware'
Found installed collection redhat.satellite:2.1.0 at '/home/pharriso/.ansible/collections/ansible_collections/redhat/satellite'
Found installed collection sensu.sensu_go:1.6.0 at '/home/pharriso/.ansible/collections/ansible_collections/sensu/sensu_go'
Found installed collection ansible.tower:3.8.2 at '/home/pharriso/.ansible/collections/ansible_collections/ansible/tower'
Found installed collection ansible.posix:1.2.0 at '/home/pharriso/.ansible/collections/ansible_collections/ansible/posix'
Found installed collection demolab.vm_config:1.0.1 at '/home/pharriso/.ansible/collections/ansible_collections/demolab/vm_config'
Found installed collection containers.podman:1.6.1 at '/home/pharriso/.ansible/collections/ansible_collections/containers/podman'
Found installed collection phoenixnap.bmc:0.6.0 at '/home/pharriso/.ansible/collections/ansible_collections/phoenixnap/bmc'
Process install dependency map
Initial connection to galaxy_server: https://hub-2.demolab.local/api/galaxy/content/rh-certified/
Found API version 'v3' with Galaxy server hub-2_rh-certified_repo (https://hub-2.demolab.local/api/galaxy/content/rh-certified/)
Opened /home/pharriso/.ansible/galaxy_token
Calling Galaxy at https://hub-2.demolab.local/api/galaxy/content/rh-certified/v3/collections/phoenixnap/bmc/
Initial connection to galaxy_server: https://hub-1.demolab.local/api/galaxy/content/rh-certified/
Opened /home/pharriso/.ansible/galaxy_token
Calling Galaxy at https://hub-1.demolab.local/api/galaxy/content/rh-certified/
Calling Galaxy at https://hub-1.demolab.local/api/galaxy/content/rh-certified/api
ERROR! Unknown error when attempting to call Galaxy at 'https://hub-1.demolab.local/api/galaxy/content/rh-certified/api': <urlopen error [Errno 113] No route to host>
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
"""Retrieve collection dependencies of a given candidate."""
        if collection_candidate.is_concrete_artifact:
            return (
                self.
                _concrete_art_mgr.
                get_direct_collection_dependencies
            )(collection_candidate)
        return (
            self.
            get_collection_version_metadata(collection_candidate).
            dependencies
        )