<!--- 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 HAS_DNF:
            return
        system_interpreters = ['/usr/libexec/platform-python',
                               '/usr/bin/python3',
                               '/usr/bin/python2',
                               '/usr/bin/python']
        if not has_respawned():
            
            interpreter = probe_interpreters_for_module(system_interpreters, 'dnf')
            if interpreter:
                
                respawn_module(interpreter)
                
        
        self.module.fail_json(
            msg="Could not import the dnf python module using {0} ({1}). "
                "Please install `python3-dnf` or `python2-dnf` package or ensure you have specified the "
                "correct ansible_python_interpreter. (attempted {2})"
                .format(sys.executable, sys.version.replace('\n', ''), system_interpreters),
            results=[]
        )
    def _configure_base(self, base, conf_file, disable_gpg_check, installroot='/'):