DM: Compute the vxlan for a given vn on demand
DM: Compute the vxlan for a given vn on demand

Currently, VN's vxlan-id is computed when there is a change in global vxlan auto mode parameter.
This fix will make ensure the compute happens only when physical router config is generated.

Change-Id: If05f5c1791b6fabaeba5d55887bfdc626f5bf496
Closes-Bug: #1515835

tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller