Use testtools.matchers.GreaterThan in test_volumes_transfer

assertGreaterEqual is not in python 2.6 and while I know the long-term
direction for Tempest is to not support 2.6, this change is
super-trivial to make this work on 2.6.

Change-Id: Ie8315b539091f5fbff4e7dfd18a9b824f81a7fb7
diff --git a/tempest/api/volume/test_volume_transfers.py b/tempest/api/volume/test_volume_transfers.py
index fc4f07d..83003b3 100644
--- a/tempest/api/volume/test_volume_transfers.py
+++ b/tempest/api/volume/test_volume_transfers.py
@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from testtools import matchers
+
 from tempest.api.volume import base
 from tempest import clients
 from tempest import config
@@ -87,7 +89,7 @@
         # or equal to 1
         resp, body = self.client.list_volume_transfers()
         self.assertEqual(200, resp.status)
-        self.assertGreaterEqual(len(body), 1)
+        self.assertThat(len(body), matchers.GreaterThan(0))
 
         # Accept a volume transfer by alt_tenant
         resp, body = self.alt_client.accept_volume_transfer(transfer_id,