QA: never put a master node offline

Before, it could happen that the master node was chosen to be set offline
in the "instance-remove-drbd-offline" test. Also changed a finally clause
to make sure that an exception while making the node online doesn't break
undoing other things.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>