bgpd: enable neighbor-nexthop-self for l2vpn evpn address family

bgpd: enable neighbor-nexthop-self for l2vpn evpn address family

In the FRR implementation of EVPN,
eBGP leaf-spine peering for EVPN is fully supported by allowing
the next hop to be propagated and not rewritten at each hop.
There are other changes also related to route import to facilitate this.
However, propagating the next hop is not correct in some cases.
Specifically, if the DC is comprised of multiple PODs
with distinct intra-POD and inter-POD VxLAN tunnels,
EVPN routes received from an adjacent POD by a border/exit leaf
must be propagated into the local POD with the next hop rewritten (to self).

Signed-off-by: Mitesh Kanjariya <mitesh@cumulusnetworks.com>