<!--- 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 not repoq:
            pkgs = []
            try:
                try:
                    pkgs = self.yum_base.returnPackagesByDep(req_spec) + \
                        self.yum_base.returnInstalledPackagesByDep(req_spec)
                except Exception as e:
                    
                    
                    
                    
                    
                    if 'repomd.xml signature could not be verified' in to_native(e):
                        if self.releasever:
                            self.module.run_command(self.yum_basecmd + ['makecache'] + ['--releasever=%s' % self.releasever])
                        else:
                            self.module.run_command(self.yum_basecmd + ['makecache'])
                        pkgs = self.yum_base.returnPackagesByDep(req_spec) + \
                            self.yum_base.returnInstalledPackagesByDep(req_spec)
                    else:
                        raise
                if not pkgs:
                    e, m, _ = self.yum_base.pkgSack.matchPackageNames([req_spec])