Use absolute import in gabbi fixture

Otherwise it gets confused with the fixture named placement.

Change-Id: I678be3af4ef70491b2a7cfd5eba2b335e207a033

placement/tests/functional/fixtures/gabbits.py

@@ -9,6 +9,7 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+from __future__ import absolute_import
 
 import os