<!--- 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 len(self.enablerepo) == 1:
            try:
                self.yum_base.repos.enableRepo(self.enablerepo[0])
            except yum.Errors.YumBaseError as e:
                if u'repository not found' in to_text(e):
                    self.module.fail_json(msg="Repository %s not found." % self.enablerepo[0])
                else:
                    raise e
        else:
            for rid in self.enablerepo:
                try:
                    self.yum_base.repos.enableRepo(rid)
                except yum.Errors.YumBaseError as e:
                    if u'repository not found' in to_text(e):
                        self.module.warn("Repository %s not found." % rid)
                    else:
                        raise e
    def is_lockfile_pid_valid(self):