Define SDC model to configure VXLAN tunnel

Define SDC model to configure VXLAN tunnel

VoLTE use case contains a VXLAN tunnel between edge and core datacenters.
This patch defines TOSCA template that enables user to input configuration
parameters required to associate local network within dataceter to a VXLAN
tunnel between data centers. Local network within data center is created
dynamically while provisioning either vIMS or vEPC services and this SDC
model enables configuration of VXLAN gateways to forward local network
traffic through inter-DC VXLAN tunnel.

Issue-Id: SDC-342

Change-Id: I1b3b8bdcff60128223d47f3bfafe50766017d2de
Signed-off-by: Arun S. Yerra <arun.yerra@huawei.com>