tests: fix and adjust topotest/bgp_aigp

tests: fix and adjust topotest/bgp_aigp

Fix and adjust the topotest post the fix for route selection with
AIGP.

When there are multiple	IGP domains (OSPF in this case), the nexthop
for a BGP route with the AIGP attribute must be resolved in its own
IGP domain.

The changes in r2/bgpd.conf and r3/bgpd.conf are needed as incorrect
IGP metrics are received from NHT for the recursive nexthops. Once
the issue is resolved, the changes can be reverted.

Signed-off-by: Enke Chen <enchen@paloaltonetworks.com>