<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
Add a --cacheonly option to the ansible yum module, similar to the --downloadonly option.

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
lib/ansible/modules/packaging/os/yum.py

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->
We have a group of Oracle servers that have no repository management server.  They update directly from Oracle's repositories.  Our thought is to do a "yum update --downloadonly" on all dev and prod servers.  Then we can update the dev servers first using "yum update --cacheonly" to test then updated and run the same command on the prod at a later time to make them consistent with the dev.
<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

<!--- HINT: You can also paste gist.github.com links for larger files -->

#
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
import os
import time
import glob
import tempfile
from abc import ABCMeta, abstractmethod
from ansible.module_utils._text import to_native
from ansible.module_utils.six import with_metaclass
yumdnf_argument_spec = dict(
    argument_spec=dict(
        allow_downgrade=dict(type='bool', default=False),
        autoremove=dict(type='bool', default=False),
        bugfix=dict(required=False, type='bool', default=False),
        conf_file=dict(type='str'),
        disable_excludes=dict(type='str', default=None),
        disable_gpg_check=dict(type='bool', default=False),
        disable_plugin=dict(type='list', elements='str', default=[]),
        disablerepo=dict(type='list', elements='str', default=[]),
        download_only=dict(type='bool', default=False),