Address i18n related deprecation warnings

Unit tests logs show trace [1]. Right now it is just
noise, but this is a time bomb waiting to blow up
into someone's face.

[1] http://logs.openstack.org/55/252755/24/check/gate-neutron-python27-constraints/d78face/console.html#_2016-02-03_03_29_52_992

Change-Id: I24d3de2ee6b1209f0e86f8fb953aab18bae8cc98

neutron/db/migration/alembic_migrations/versions/mitaka/expand/1df244e556f5_add_unique_ha_router_agent_port_bindings.py

@@ -28,6 +28,8 @@
 from alembic import op
 import sqlalchemy as sa
 
+from neutron._i18n import _
+
 from neutron.common import exceptions
 
 UNIQUE_NAME = 'uniq_ha_router_agent_port_bindings0port_id0l3_agent_id'