Service instance SNAT in network namespace

Add service instance hypervisor type in schema to distinguish VM to
network namespace instance. Also add a new service type for source NAT
service.

When the service instance hypervisor type is set to 'netns' and the
service type is 'source-nat', the SVC monitor creates virtual machine(s)
and virtual machines interface between left and right virtual networks
and schedules all of them on different virtual router(s).

Change-Id: I7eb8c21a8621731f42e0b7fa4b83241b602f273a
Signed-off-by: Édouard Thuleau <edouard.thuleau@cloudwatt.com>