Avoid performing extra query for fetching qos bindings

Bug 1215872

Add a relationship with eager loading in the Port and Network models,
thus preventing the 'extend' functions from performing extra queries.
This patch also slight alters the methods for processing qos_queue
bindings in order to allow them for populating the qos_queue id in
the response being created.

Change-Id: Ie80d12aeed5de94afa61f23d7bcfc21372f23c4f
pass default l3_gateway_service_uuid if not specified

Previously, if one created an external network of type l3_ext and did
not specify the default_gw_service_uuid via --provider:physical_network
an exception would be raised on the backend as we need to specify the
l3_gw_service_uuid we want nvp to use.

Fixes bug: 1224660

Change-Id: I6312956528c17d6a704d4e4ef629916f6463312a
Merge "Avoid performing extra query for fetching qos bindings"
Merge "pass default l3_gateway_service_uuid if not specified"