<!--- 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 -->

response['results'].append("{0}: {1}".format(install_action, package))
                for package in self.base.transaction.remove_set:
                    response['results'].append("Removed: {0}".format(package))
                if failure_response['failures']:
                    failure_response['msg'] = 'Failed to install some of the specified packages'
                    self.module.fail_json(**failure_response)
                if self.module.check_mode:
                    response['msg'] = "Check mode: No changes made, but would have if not in check mode"
                    self.module.exit_json(**response)
                try:
                    if self.download_only and self.download_dir and self.base.conf.destdir:
                        dnf.util.ensure_dir(self.base.conf.destdir)
                        self.base.repos.all().pkgdir = self.base.conf.destdir
                    self.base.download_packages(self.base.transaction.install_set)
                except dnf.exceptions.DownloadError as e:
                    self.module.fail_json(
                        msg="Failed to download packages: {0}".format(to_text(e)),
                        results=[],
                    )
                
                
                if not self.disable_gpg_check:
                    for package in self.base.transaction.install_set:
                        fail = False
                        gpgres, gpgerr = self.base._sig_check_pkg(package)
                        if gpgres == 0:  
                            continue
                        elif gpgres == 1:  
                            try:
                                self.base._get_key_for_package(package)