[SNAT SVC monitor] Set the left interface virtual network

The left interface of a NetNS SI for source NAT is attach to a dedicated VN.
And that VN is buid by the service monitor code, so that code sould also update
the SI property to correctly point to that created VN.

Change-Id: I96f2c0224e0231dae9d1ab417f897f3636306b08
Merge "[SNAT SVC monitor] Set the left interface virtual network"