Add list-key-ids command
This will be used by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2240 so we can remove keys that should already have been deleted from the authorized_keys
file.
This command is intended to be called from Gitlab::Shell
in the Rails app so we can check if the IDs exist in the DB. If they don't, then rm-key
will be called from the Rails app.