<!--- 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 -->

from ansible.module_utils.common.respawn import has_respawned, respawn_module
from ansible.module_utils._text import to_native, to_text
from ansible.module_utils.urls import fetch_url
from ansible.module_utils.yumdnf import YumDnf, yumdnf_argument_spec
import errno
import os
import re
import sys
import tempfile
try:
    import rpm
    HAS_RPM_PYTHON = True
except ImportError:
    HAS_RPM_PYTHON = False
try:
    import yum
    HAS_YUM_PYTHON = True
except ImportError:
    HAS_YUM_PYTHON = False
try:
    from yum.misc import find_unfinished_transactions, find_ts_remaining
    from rpmUtils.miscutils import splitFilename, compareEVR
    transaction_helpers = True
except ImportError:
    transaction_helpers = False
from contextlib import contextmanager
from ansible.module_utils.urls import fetch_file
def_qf = "%{epoch}:%{name}-%{version}-%{release}.%{arch}"
rpmbin = None
class YumModule(YumDnf):