<!--- 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 -->

failure_response['failures'].append(' '.join((module, to_native(e))))
                for group in groups:
                    try:
                        try:
                            self.base.group_upgrade(group)
                            response['results'].append("Group {0} upgraded.".format(group))
                        except dnf.exceptions.CompsError:
                            if not self.update_only:
                                
                                group_pkg_count_installed = self.base.group_install(group, dnf.const.GROUP_PACKAGE_TYPES)
                                if group_pkg_count_installed == 0:
                                    response['results'].append("Group {0} already installed.".format(group))
                                else:
                                    response['results'].append("Group {0} installed.".format(group))
                    except dnf.exceptions.Error as e:
                        failure_response['failures'].append(" ".join((group, to_native(e))))
                for environment in environments:
                    try:
                        try:
                            self.base.environment_upgrade(environment)
                        except dnf.exceptions.CompsError:
                            
                            self.base.environment_install(environment, dnf.const.GROUP_PACKAGE_TYPES)
                    except dnf.exceptions.DepsolveError as e:
                        failure_response['msg'] = "Depsolve Error occured attempting to install environment: {0}".format(environment)
                    except dnf.exceptions.Error as e:
                        failure_response['failures'].append(" ".join((environment, to_native(e))))
                if self.update_only:
                    not_installed = self._update_only(pkg_specs)
                    for spec in not_installed: