zebra, bgpd: Use L3 interface for VRF's VNI in route install

zebra, bgpd: Use L3 interface for VRF's VNI in route install

In the case of EVPN symmetric routing, the tenant VRF is associated with
a VNI that is used for routing and commonly referred to as the L3 VNI or
VRF VNI. Corresponding to this VNI is a VLAN and its associated L3 (IP)
interface (SVI). Overlay next hops (i.e., next hops for routes in the
tenant VRF) are reachable over this interface.

https://tools.ietf.org/html/draft-ietf-bess-evpn-prefix-advertisement
section 4.4 provides additional description of the above constructs.

Use the L3 interface exchanged between zebra and bgp in route install.
This patch in conjunction with the earlier one helps to eliminate some
special code in zebra to derive the next hop's interface.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>