Re-home neutron-lib constants

Some constants have been re-homed to neutron-lib, and will soon be
removed from neutron itself.  This change set alters imports accordingly
in anticipation.

Change-Id: Icdeabcc504ebcf3a0823587b042595ede420ce84

networking_powervm/plugins/ml2/drivers/mech_pvm_sea.py

@@ -14,8 +14,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from neutron.plugins.common import constants as p_constants
 from neutron.plugins.ml2 import driver_api as api
+from neutron_lib import constants as p_constants
 from oslo_log import log
 
 from networking_powervm.plugins.ibm.agent.powervm import constants as pconst

networking_powervm/plugins/ml2/drivers/mech_pvm_sriov.py

@@ -14,8 +14,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from neutron.plugins.common import constants as p_constants
 from neutron_lib.api.definitions import portbindings
+from neutron_lib import constants as p_constants
 from oslo_log import log
 
 from networking_powervm.plugins.ibm.agent.powervm import constants as pconst