This ensures that instance.availability_zone reflects what AZ the
instance is actually in. In the case where no AZ was requested at
boot, previously this would always be some default value, which
isn't as useful to the things that need to consider what AZ the
instance is actually in (without doing all the aggregate math to
determine it on the fly).
Related to blueprint cells-aware-api
Change-Id: I8d426f2635232ffc4b510548a905794ca88d7f99
Dan Smith