tearDown tmp_volumes array itself in VolumeTest

Each volume in the array holds a reference to a docker.client.Client object and
therefore a connection pool which leaves fds open once the test has completed.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

tests/integration/volume_test.py

@@ -17,6 +17,7 @@ class VolumeTest(DockerClientTestCase):
                 self.client.remove_volume(volume.full_name)
             except DockerException:
                 pass
+        del self.tmp_volumes
 
     def create_volume(self, name, driver=None, opts=None, external=None):
         if external and isinstance(external, bool):