lbaas on a network without gateway

Not only check the gateway_ip in subnet, but also
check the host_routes. If there is a default route in host_routes
then the next hop of the default route is also a gateway_ip.

Closes-Bug: 1300017
DocImpact

Change-Id: Id3ae04bfa6cefb1c030857894f0594828b8e856d
Merge "lbaas on a network without gateway"