Add an update script to handle version upgrades

[thatcherhubbard]

The only value in the state of a ROSA cluster that should trigger a TF update action should be the OpenShift version, because version upgrades can be done with the rosa CLI.

This will require:

• An update script that is triggered by a change to the configured version
• Stripping the Version key and sub-keys from the read action so if a cluster gets upgraded automatically or via the console it doesn't break TF
• A validation to make sure a newly configured version number matches the available-upgrades key in the cluster state