<!--- 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 -->

w,
                            '%s.%s from %s' % (
                                pkg['version'],
                                pkg['dist'],
                                pkg['repo']
                            )
                        )
                    )
        if self.update_only:
            res['changes'] = dict(installed=[], updated=to_update)
        else:
            res['changes'] = dict(installed=pkgs['install'], updated=to_update)
        if obsoletes:
            res['obsoletes'] = obsoletes
        
        if self.module.check_mode:
            if will_update or pkgs['install']:
                res['changed'] = True
            return res
        if self.releasever:
            cmd.extend(['--releasever=%s' % self.releasever])
        
        if cmd:     
            rc, out, err = self.module.run_command(cmd)
            res['changed'] = True
        elif self.update_only:
            if pkgs['update']:
                cmd = self.yum_basecmd + ['update'] + pkgs['update']
                lang_env = dict(LANG='C', LC_ALL='C', LC_MESSAGES='C')
                rc, out, err = self.module.run_command(cmd, environ_update=lang_env)