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/admin/test_multi_backend.py b/tempest/api/volume/admin/test_multi_backend.py
index b8047b2..c7e989d 100644
--- a/tempest/api/volume/admin/test_multi_backend.py
+++ b/tempest/api/volume/admin/test_multi_backend.py
@@ -11,9 +11,9 @@
 #    under the License.
 
 from oslo_log import log as logging
-from tempest_lib.common.utils import data_utils
 
 from tempest.api.volume import base
+from tempest.common.utils import data_utils
 from tempest import config
 from tempest import test