bgpd: CLI to advertise gateway IP overlay index

bgpd: CLI to advertise gateway IP overlay index

Adds gateway-ip option to advertise ipv4/ipv6 unicast CLI.

dev(config-router-af)# advertise <ipv4|ipv6> unicast
  <cr>
  gateway-ip      Specify EVPN Overlay Index
  route-map       route-map for filtering specific routes

When gateway-ip is specified, gateway IP field of EVPN RT-5 NLRI is filled with
the BGP nexthop of the vrf prefix being advertised.

No support for ESI overlay index yet.

Test cases:
1) advertise ipv4 unicast
2) advertise ipv4 unicast gateway-ip
3) advertise ipv6 unicast
4) advertise ipv6 unicast gateway-ip
5) Modify from no-overlay-index to gateway-ip
6) Modify from gateway-ip to no-overlay-index
7) CLI with route-map and modify route-map

Author: Sri Mohana Singamsetty <srimohans@gmail.com>
Signed-off-by: Sri Mohana Singamsetty <srimohans@gmail.com>