select router with subnet's gateway_ip for floatingip

select router with subnet's gateway_ip for floatingip

1) when a subnet is connected to multiple routers and
all these routers are connected to same external network,
then select the router with subnet's gateway_ip, if available,
for managing floatingip.

2) Otherwise go with default existing behavior i.e
select first router in internal subnet, that also present on external network.

For scenario 1), if the router with gateway ip not selected,
then for connections initiated by external agent towards floatingip
won't get response with floatingip as source address,
instead gw ip of router(i.e router with subnet's gateway_ip) as source.
Details about the bug at [1]

[1] https://bugs.launchpad.net/neutron/+bug/1470765/comments/4

Closes-bug: #1470765
Change-Id: If054945eab058c7138aabbb22cda15890ccb502c