zebra: maintain mcast tunnel origination and termination SG entries

zebra: maintain mcast tunnel origination and termination SG entries

Each multicast tunnel is associated with a -
1. Tunnel origination mroute that is used for forwarding the
VxLAN encapsulated flow -
S - local VTEP-IP
G - BUM mcast-group
2. And a tunnel termination entry -
S - * (any remote VTEP)
G - BUM mcast-group

Multiple L2 VNIs can share the same BUM mcast group (and local-VTEP-IP).
Zebra maintains an mcast (SG) hash table to pass this info to pimd for
subsequent MDT setup.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>