Allocation API: use generate_uuid for random resource classes

We see clashes with using random numbers, switch to UUIDs instead.

Change-Id: I8a529bcfd319a7f0edbdd45727cef7efe6f2f8cb
diff --git a/ironic_tempest_plugin/tests/api/admin/test_allocations.py b/ironic_tempest_plugin/tests/api/admin/test_allocations.py
index 4e48374..a6e8e9c 100644
--- a/ironic_tempest_plugin/tests/api/admin/test_allocations.py
+++ b/ironic_tempest_plugin/tests/api/admin/test_allocations.py
@@ -10,8 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import random
-
+from oslo_utils import uuidutils
 from tempest import config
 from tempest.lib import decorators
 from tempest.lib import exceptions as lib_exc
@@ -37,7 +36,7 @@
 
         # Generate a resource class to prevent parallel tests from clashing
         # with each other.
-        self.resource_class = 'x-small-%d' % random.randrange(1024)
+        self.resource_class = uuidutils.generate_uuid()
 
         _, self.chassis = self.create_chassis()
         _, self.node = self.create_node(self.chassis['uuid'],