use l3 api def from neutron-lib
use l3 api def from neutron-lib

Commit I81748aa0e48b1275df3e1ea41b1d36a117d0097d added the l3 extension
API definition to neutron-lib, commit
I2324a3a02789c798248cab41c278a2d9981d24be rehomed the l3 exceptions and
Ifd79eb1a92853e49bd4ef028e7a7bd89811c6957 shims the l3 exceptions in
neutron.

This patch consumes the l3 API definition from neutron-lib
in prep for If2e66e06b83e15ee2851ea2bc3b64ad366e675dd
by:
- Using the L3 update_attributes_map method to add the l3 ext gateway
attributes rather than accessing them directly.
- Moving the former after the _setUpExtension method is called as the
fixture for attribute maps is setup therein.
- Using neutron-lib's l3 API definition where applicable.

Change-Id: I24569675f00e5a856cd0f873febede34af16e96a

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl