zebra: header changes for l2 vni bum-mcast-grp handling

zebra: header changes for l2 vni bum-mcast-grp handling

The multicast group ip address for BUM traffic is configurable per-l2-vni.
One way to configure that is to setup a vxlan device that per-l2-vni and
specify the address against that vxlan device -
root@TORS1:~# vtysh -c "show interface vx-1000" |grep -i vxlan
  Interface Type Vxlan
  VxLAN Id 1000 VTEP IP: 27.0.0.15 Access VLAN Id 1000 Mcast 239.1.1.100
root@TORS1:~# vtysh -c "show evpn vni 1000" |grep Mcast
 Mcast group: 239.1.1.100
root@TORS1:~#

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