QA: Run cluster-verify after instance tests

We want to make sure that instance operations don't leave the cluster in a
bad state. The "instance-remove-drbd-offline" test leaves some debris
behind, so it's been moved to the last position.

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