zebra: allocate one nexthop id per-VTEP instead of one per-ES-VTEP

zebra: allocate one nexthop id per-VTEP instead of one per-ES-VTEP

This is an optimization to reduce the number of L2 nexthops. A
l2 or fdb nexthop simply provides the dataplane with a nexthop ip-
torm-12:mgmt:~# ip nexthop
id 268435461 via 27.0.0.20 scope link fdb
id 268435463 via 27.0.0.20 scope link fdb
id 268435465 via 27.0.0.20 scope link fdb

So there is no need to allocate a nexthop per-ES/per-VTEP. There
can be 100+ ESs per-VTEP so this change cuts the scale down by a
factor of 100.

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