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: I420f1697420bffb5d97c6db92f2db5bc17462acc
Closes-Bug: #1398275