Enable py34 tests for pluggable ipam backend

Enable neutron.tests.unit.db.test_ipam_pluggable_backend.
It was disabled after extending ipam tests to run db_base_plugin tests,
which are not py34 compatible yet.

Change-Id: I3ae491fa79d4c3311a86e98db3fb2f7d5926a0ba

tox.ini

@@ -164,6 +164,7 @@ commands = python setup.py test --testr-args='{posargs: \
      neutron.tests.unit.db.test_ipam_backend_mixin \
      neutron.tests.unit.db.test_l3_dvr_db \
      neutron.tests.unit.db.test_l3_hamode_db \
+     neutron.tests.unit.db.test_ipam_pluggable_backend \
      neutron.tests.unit.db.test_migration \
      neutron.tests.unit.db.test_agents_db \
      neutron.tests.unit.db.quota.test_api \