Support multi-gateway NS networking with EW enabled

In multi-region cloud deployment, a requirement is that each
OpenStack cloud provides external network, north-south traffic
is expected to be handled locally for shortest path, and/or
use multiple external networks to help application north-south
traffic redundancy, at the same time east-west networking of
tenant's networks between OpenStack cloud is also needed.
This path will introduce east-west gateway router to address
the east-west networking for local networks in different
OpenStack clouds.