Add VPCZoneIdentifier attribute for autoscaling

For now, only one subnet can be specified in the launch configuration because
Neutron doesn't have the ability to map subnets with availability zones.
See bug #1096017 for details.

Change-Id: I46ef4d17b2109c97664f7606c97b16071553ebcd
Fixes: bug #1196494
Merge "Add VPCZoneIdentifier attribute for autoscaling"