<!--- 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 HAS_DNF
    def run(self):
        """The main function."""
        
        if self.autoremove:
            if LooseVersion(dnf.__version__) < LooseVersion('2.0.1'):
                self.module.fail_json(
                    msg="Autoremove requires dnf>=2.0.1. Current dnf version is %s" % dnf.__version__,
                    results=[],
                )
        
        if self.download_dir:
            if LooseVersion(dnf.__version__) < LooseVersion('2.6.2'):
                self.module.fail_json(
                    msg="download_dir requires dnf>=2.6.2. Current dnf version is %s" % dnf.__version__,
                    results=[],
                )
        if self.update_cache and not self.names and not self.list:
            self.base = self._base(
                self.conf_file, self.disable_gpg_check, self.disablerepo,
                self.enablerepo, self.installroot
            )
            self.module.exit_json(
                msg="Cache updated",
                changed=False,