Add compute hosts to their availability zone

There's a new crowbar-nova-set-availability-zone utility that will be
run on the controller node to configure the availability zone of compute
nodes. It will only do so when the compute nodes are known to the
controller nodes, which might happen after the first chef-client run;
but 15 minutes later, everything will go fine.
Do not move to default AZ nodes that are in non-crowbar AZ

If the cloud admin manually creates AZ, then we don't want crowbar to
override that. So nodes that have no configured AZ will stay in a
non-crowbar AZ if they're configured so.
Add "-AZ" to crowbar prefix for host aggregate names

Crowbar might create non-AZ related host aggregates in the future...
Merge pull request #293 from vuntz/az

Add compute hosts to their availability zone