<!--- 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 -->

if repoq:
                repoq += ['-c', self.conf_file]
        if self.skip_broken:
            self.yum_basecmd.extend(['--skip-broken'])
        if self.disablerepo:
            self.yum_basecmd.extend(['--disablerepo=%s' % ','.join(self.disablerepo)])
        if self.enablerepo:
            self.yum_basecmd.extend(['--enablerepo=%s' % ','.join(self.enablerepo)])
        if self.enable_plugin:
            self.yum_basecmd.extend(['--enableplugin', ','.join(self.enable_plugin)])
        if self.disable_plugin:
            self.yum_basecmd.extend(['--disableplugin', ','.join(self.disable_plugin)])
        if self.exclude:
            e_cmd = ['--exclude=%s' % ','.join(self.exclude)]
            self.yum_basecmd.extend(e_cmd)
        if self.disable_excludes:
            self.yum_basecmd.extend(['--disableexcludes=%s' % self.disable_excludes])
        if self.download_only:
            self.yum_basecmd.extend(['--downloadonly'])
            if self.download_dir:
                self.yum_basecmd.extend(['--downloaddir=%s' % self.download_dir])
        if self.releasever:
            self.yum_basecmd.extend(['--releasever=%s' % self.releasever])
        if self.installroot != '/':
            
            
            
            e_cmd = ['--installroot=%s' % self.installroot]
            self.yum_basecmd.extend(e_cmd)
        if self.state in ('installed', 'present', 'latest'):