Use the prefix-embedded rand_name method

A version of data_utils.rand_name in tempest_lib was created in tempest to
include a prefix specified in the conf file. This commit changes the
data_utils imports to use the new one in tempest.

Change-Id: Ie3fb5c3ac9afe365bf24e40b00be88d66da7c507
diff --git a/tempest/api/volume/test_qos.py b/tempest/api/volume/test_qos.py
index 863a698..84fd7f6 100644
--- a/tempest/api/volume/test_qos.py
+++ b/tempest/api/volume/test_qos.py
@@ -12,9 +12,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from tempest_lib.common.utils import data_utils as utils
-
 from tempest.api.volume import base
+from tempest.common.utils import data_utils as utils
 from tempest import test