Restore UUID and human-ID bash completion

The refactoring to use Oslo's `Resource` broke UUID and human_id
(https://review.openstack.org/#/c/63104/) bash-completion by removing the code
that writes the IDs to the cache.

This patch adds the missing code back, careful to handle cases where the
`Manager` in a particular client doesn't yet support the
`write_to_completion_cache` method that Novaclient does.

Change-Id: I7b231eb492509cdfc844e7ed036892e37c5bb8b6
Closes-Bug: 1327431
Merge "Restore UUID and human-ID bash completion"