### Summary

`ansible-galaxy collection install` makes a lot of requests to galaxy.ansible.com and can occasionally hit the rate limit. When that happens, galaxy.ansible.com will return a 520 http code and `ansible-galaxy` fails to download the collection and exits. Ideally when the client encounters a rate limiting http code (either 520, or 429), it should wait, slow down the request rate and try again rather than exiting.

More information is available here: https://github.com/ansible/galaxy/issues/2429 

### Issue Type

Bug Report

### Component Name

ansible-galaxy

### Ansible Version

```console
$ ansible --version
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible --version
ansible 2.10.7
  config file = None
  configured module search path = ['/root/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.8/dist-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0]
```


### Configuration

```console
$ ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~#
```


### OS / Environment

All

### Steps to Reproduce

Run `ansible-galaxy collection install `amazon.aws`. If your internet is fast enough, this will occasionally fail when galaxy.ansible.com returns a 520 error code.

### Expected Results

Collection should be installed.

### Actual Results

```console
`ansible-galaxy` encounters a 429 or 520 http code and exits.
```


### Code of Conduct

I agree to follow the Ansible Code of Conduct


api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/")
    actual = {'Authorization': 'Custom token'}
    api._add_auth_token(actual, "", required=True)
    assert actual == {'Authorization': 'Custom token'}
def test_initialise_galaxy(monkeypatch):
    mock_open = MagicMock()
    mock_open.side_effect = [
        StringIO(u'{"available_versions":{"v1":"v1/"}}'),
        StringIO(u'{"token":"my token"}'),
    ]
    monkeypatch.setattr(galaxy_api, 'open_url', mock_open)
    api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/")
    actual = api.authenticate("github_token")
    assert len(api.available_api_versions) == 2
    assert api.available_api_versions['v1'] == u'v1/'
    assert api.available_api_versions['v2'] == u'v2/'
    assert actual == {u'token': u'my token'}
    assert mock_open.call_count == 2
    assert mock_open.mock_calls[0][1 'https://galaxy.ansible.com/api/'
    assert 'ansible-galaxy' in mock_open.mock_calls[0][2]['http_agent']
    assert mock_open.mock_calls[1][1 'https://galaxy.ansible.com/api/v1/tokens/'
    assert 'ansible-galaxy' in mock_open.mock_calls[1][2]['http_agent']
    assert mock_open.mock_calls[1][2]['data'] == 'github_token=github_token'
def test_initialise_galaxy_with_auth(monkeypatch):
    mock_open = MagicMock()
    mock_open.side_effect = [
        StringIO(u'{"available_versions":{"v1":"v1/"}}'),
        StringIO(u'{"token":"my token"}'),
    ]
    monkeypatch.setattr(galaxy_api, 'open_url', mock_open)