Fix bug detach volume fails with "KeyError" in EC2

Modified volume['instance_uuid'] to volume.get('instance_uuid'),
to avoid "KeyError", when detach a volume with "attaching" status.

Change-Id: Ic1aacab212c2a2026d18ef57ad9d77fdc6d86b47
Closes-Bug: #1280532
Merge "Fix bug detach volume fails with "KeyError" in EC2"