### 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


responses = get_collection_versions()
    cache_file = os.path.join(cache_dir, 'api.json')
    api = get_test_galaxy_api('https://ga.com/api/', 'v2', no_cache=False)
    mock_open = MagicMock(
        side_effect=[
            StringIO(to_text(json.dumps(r)))
            for r in responses
        ]
    )
    monkeypatch.setattr(galaxy_api, 'open_url', mock_open)
    actual_versions = api.get_collection_versions('namespace', 'collection')
    assert actual_versions == [u'1.0.0', u'1.0.1', u'1.0.2', u'1.0.3', u'1.0.4', u'1.0.5']
    with open(cache_file) as fd:
        final_cache = json.loads(fd.read())
    cached_server = final_cache['ga.com:']
    cached_collection = cached_server['/api/v2/collections/namespace/collection/versions/']
    cached_versions = [r['version'] for r in cached_collection['results']]
    assert final_cache == api._cache
    assert cached_versions == actual_versions
def test_cache_flaky_pagination(cache_dir, monkeypatch):
    responses = get_collection_versions()
    cache_file = os.path.join(cache_dir, 'api.json')
    api = get_test_galaxy_api('https://ga.com/api/', 'v2', no_cache=False)
    # First attempt, fail midway through
    mock_open = MagicMock(
        side_effect=[
            StringIO(to_text(json.dumps(responses[0]))),
            StringIO(to_text(json.dumps(responses[1]))),
            urllib_error.HTTPError(responses[1]['next'], 500, 'Error', {}, StringIO()),
            StringIO(to_text(json.dumps(responses[3]))),