<!--- 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 -->

conf.substitutions['releasever'] = self.releasever
        
        if self.skip_broken:
            conf.strict = 0
        
        if self.nobest:
            conf.best = 0
        if self.download_only:
            conf.downloadonly = True
            if self.download_dir:
                conf.destdir = self.download_dir
        
        conf.clean_requirements_on_remove = self.autoremove
        
        conf.install_weak_deps = self.install_weak_deps
    def _specify_repositories(self, base, disablerepo, enablerepo):
        """Enable and disable repositories matching the provided patterns."""
        base.read_all_repos()
        repos = base.repos
        
        for repo_pattern in disablerepo:
            if repo_pattern:
                for repo in repos.get_matching(repo_pattern):
                    repo.disable()
        
        for repo_pattern in enablerepo:
            if repo_pattern:
                for repo in repos.get_matching(repo_pattern):
                    repo.enable()
    def _base(self, conf_file, disable_gpg_check, disablerepo, enablerepo, installroot):