Fix instance AZ check when attaching volumes

Closes-Bug: #1255347

Nova was incorrectly using the instance['availability_zone']
attribute, to get the AZ of an instance use the
get_instance_availability_zone method.

Change-Id: Ied912c171100a450754d315027bf5c407ac067bb
Fix up cells scheduling with AZs

Change-Id: I0e7936ba5cc1fe745f18719424f752e91a6aa520