Ignore failures while shutting down instances during failover from offline node

Don't abort failover if instance shutdown doesn't work on a node marked
offline. The node is offline, so the instances living on it are too. Before
you had to use --ignore-consistency to archieve that.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>