<!--- 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 -->

- name: Install the latest version of Apache and MariaDB
  dnf:
    name:
      - httpd
      - mariadb-server
    state: latest
- name: Remove the Apache package
  dnf:
    name: httpd
    state: absent
- name: Install the latest version of Apache from the testing repo
  dnf:
    name: httpd
    enablerepo: testing
    state: present
- name: Upgrade all packages
  dnf:
    name: "*"
    state: latest
- name: Install the nginx rpm from a remote repo
  dnf:
    name: 'http://nginx.org/packages/centos/6/noarch/RPMS/nginx-release-centos-6-0.el6.ngx.noarch.rpm'
    state: present
- name: Install nginx rpm from a local file
  dnf:
    name: /usr/local/src/nginx-release-centos-6-0.el6.ngx.noarch.rpm
    state: present
- name: Install the 'Development tools' package group
  dnf:
    name: '@Development tools'