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.
(cherry picked from commit