Add netns white-list compute nodes

Use availability-zone with aggregate hosts to define
a set of white-list compute nodes where LB or SNAT
network namespaces can be launched.

User can configure the zone in contrail-svc-monitor.conf
netns_availability_zone = <name of avail zone>

Change-Id: I19303538bb79f67f2936190e95c8a753ad24555a
Merge "Add netns white-list compute nodes"