<!--- 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 -->

"""List package info based on the command."""
        
        if command == 'updates':
            command = 'upgrades'
        
        if command in ['installed', 'upgrades', 'available']:
            results = [
                self._package_dict(package)
                for package in getattr(self.base.sack.query(), command)()]
        
        elif command in ['repos', 'repositories']:
            results = [
                {'repoid': repo.id, 'state': 'enabled'}
                for repo in self.base.repos.iter_enabled()]
        
        else:
            packages = dnf.subject.Subject(command).get_best_query(self.base.sack)
            results = [self._package_dict(package) for package in packages]
        self.module.exit_json(msg="", results=results)
    def _is_installed(self, pkg):