Fix tempest router migration test when HA enabled
Fix tempest router migration test when HA enabled

When run in an HA or DVR configured environment,
the test_update_router_extra_attributes_bumps_revision
API test can fail if the release does not support
router migration from CVR-HA to DVR.

Although Ocata and later releases support any type of
router migration, older ones do not, so the test
should be explicit and test a known valid migration.

Change-Id: I8caf86817a5cd8e4d0cf05d7beef850756e84bab
Related-bug: #1679794

mahak/neutron,huntxu/neutron,eayunstack/neutron,huntxu/neutron,mahak/neutron,mahak/neutron,openstack/neutron,noironetworks/neutron,openstack/neutron,openstack/neutron,noironetworks/neutron,eayunstack/neutron