SubnetRoute enhancements to VPN Service models

This commit deals with changes required to construct
internal data structures to provide SubnetRoute
facility in VPN Service.  With SubnetRoute facility
all packets intended for a cloud coming from a
DC gateway will be punted to one DPN that will be
considered as designated DPN.

Change-Id: Ie33ad0c62bf686496460b94bd4de6f9e28e1ae5b
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>
Co-Authored-By: Vivekanandan Narasimhan <n.vivekanandan@ericsson.com>
Co-Authored-By: Karan Raj Singh <karan.raj.s.singh@ericsson.com>
Co-Authored-By: Achuth Maniyedath <achuth.maniyedath@ericsson.com>