DM: Default values for VN Forwarding mode
DM: Default values for VN Forwarding mode

If VN forwarding mode is not set, then global forwarding mode is consulted
and if global forwarding mode is not set, then it is assumed 'l2_l3' mode.

Change-Id: I33cec8f85e545ed9cd705998ce753e7573a7bfb0
Closes-Bug: #1514966

eonpatapon/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,sajuptpm/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,tcpcloud/contrail-controller,sajuptpm/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,sajuptpm/contrail-controller,sajuptpm/contrail-controller,eonpatapon/contrail-controller,tcpcloud/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,sajuptpm/contrail-controller,codilime/contrail-controller,tcpcloud/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,sajuptpm/contrail-controller,codilime/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller