Add rc code to docker_creds module

[michaelgugino]

This commit ensures that successful attempts at creating
registry credentials returns an rc == 0.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1602120

[mtnbikenc]

In order for a task to effectively use 'rc', this module will also need to return 'rc' from each instance of module.fail_json as well.

Alternatively, to use retries with this module, the (three) tasks that use this module could be updated to use the failed status which is already reported.

  until: not (foo_oreg_auth_credentials_create is failed)

[michaelgugino]

@mtnbikenc I tried with forcing it to fail by a bad set of credentials, it works as expected; module.fail_json provides the rc=1 condition by default.

FAILED - RETRYING: Create credentials for docker cli registry auth (alternative) (1 retries left).Result was: {
    "attempts": 3,
    "changed": false,
    "module_stderr": "Traceback (most recent call last):\n  File \"/tmp/ansible_qutONp/ansible_module_docker_creds.py\", line 237, in <module>\n    main()\n  File \"/tmp/ansible_qutONp/ansible_module_docker_creds.py\", line 233, in main\n    run_module()\n  File \"/tmp/ansible_qutONp/ansible_module_docker_creds.py\", line 219, in run_module\n    validate_registry_login(module, registry, encoded_auth)\n  File \"/tmp/ansible_qutONp/ansible_module_docker_creds.py\", line 141, in validate_registry_login\n    urllib.request.urlopen(req)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 154, in urlopen\n    return opener.open(url, data, timeout)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 429, in open\n    response = self._open(req, data)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 447, in _open\n    '_open', req)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 407, in _call_chain\n    result = func(*args)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 1243, in https_open\n    context=self._context)\n  File \"/usr/lib64/python2.7/urllib2.py\", line 1200, in do_open\n    raise URLError(err)\nurllib2.URLError: <urlopen error [Errno -2] Name or service not known>\n",
    "module_stdout": "",
    "msg": "MODULE FAILURE",
    "rc": 1,
    "retries": 4
}

[mtnbikenc]

/lgtm

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