<!--- 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 -->

except dnf.exceptions.Error as e:
                                fail = True
                        else:  
                            fail = True
                        if fail:
                            msg = 'Failed to validate GPG signature for {0}'.format(package)
                            self.module.fail_json(msg)
                if self.download_only:
                    
                    
                    self.module.exit_json(**response)
                else:
                    self.base.do_transaction()
                if failure_response['failures']:
                    failure_response['msg'] = 'Failed to install some of the specified packages'
                    self.module.exit_json(**response)
                self.module.exit_json(**response)
        except dnf.exceptions.DepsolveError as e:
            failure_response['msg'] = "Depsolve Error occured: {0}".format(to_native(e))
            self.module.fail_json(**failure_response)
        except dnf.exceptions.Error as e:
            if to_text("already installed") in to_text(e):
                response['changed'] = False
                response['results'].append("Package already installed: {0}".format(to_native(e)))
                self.module.exit_json(**response)
            else:
                failure_response['msg'] = "Unknown Error occured: {0}".format(to_native(e))
                self.module.fail_json(**failure_response)
    @staticmethod
    def has_dnf():