QA: Clean up after "instance-remove-drbd-offline" test

This test used to leave a node with orphan logical volumes and unused DRBD
minors. Now, the expected leftovers are cleaned up after the test, and
cluster-verify is run.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>