Fix L3 HA network creation to allow user to create router

Fix L3 HA network creation to allow user to create router

Update HA Network creation to use an admin context to allow Neutron
to create the tenant-less network required for the HA router when
it does not yet exist and is being created by a non-admin user.

Neutron creates these resources without a tenant so users cannot see
or modify the HA network, ports, etc.  Port creation and association
already use elivated admin contexts to allow their function when
an user attempts to create a HA L3 router.

Conflicts:
        neutron/tests/unit/db/test_l3_ha_db.py

Change-Id: I36166158a0970b8d08d6702054b11a43fb684281
Closes-Bug: #1388716
(cherry picked from commit cc9bc24229f1d79dc99303db1affc03c030c011e)