<!--- 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 -->

"""Return a fully configured dnf Base object."""
        base = dnf.Base()
        self._configure_base(base, conf_file, disable_gpg_check, installroot)
        try:
            
            
            base.setup_loggers()
        except AttributeError:
            pass
        try:
            base.init_plugins(set(self.disable_plugin), set(self.enable_plugin))
            base.pre_configure_plugins()
        except AttributeError:
            pass  
        self._specify_repositories(base, disablerepo, enablerepo)
        try:
            base.configure_plugins()