Attach volume eventhough the instance is shutoff

It should be possible to attach a volume to a shutoff instance.
Nova CLI (nova volume-attach) allows this and Horizon should be
on par.

Change-Id: Ie7c351e5b23f218d6d10c80b9aebbf233c62b7be
Closes-Bug: 1251255
Merge "Attach volume eventhough the instance is shutoff"