Eliminate extra queries used to retrieve gw_ports

Eliminate extra queries used to retrieve gw_ports

The _get_sync_routers method was calling get_routers and
then getting the gateway ports from the db in a separate
get_ports call. This extra call is unnecessary since is
already an SQL relationship directly between the router
and it's gw_port.

This patch eliminates all of the additional gw_port retrieval
logic by replacing the get_routers call with a _get_collection
call to make use of the gw_port object already present on
each router object.

Change-Id: I478bfef8b0273b343aa72bcd6787a486eba4f006
Partial-Bug: #1445412