Allocate an IP address from each subnet for service node (dns_server_address)
during subnet create. Also delete the address when subnet is deleted.

Fix interface_ksync - dont block interface creation l2_inactive.
Add walker for physical device VN in multicast. This is required to do walk again when VN has VRF available.
There may be case where physicaldevices-vn notification may have a VN without VRF.