Make isolated volume tests have unique tenant

The isolated credential volume tests need to have unique tenant
names otherwise when the test_volume_types_extra_specs tests are
run in parallel they step on each other.

Change-Id: I7f6d81af97cd48522ea52477a3afe9564f717d89
diff --git a/tempest/tests/volume/base.py b/tempest/tests/volume/base.py
index 32c211e..37664ec 100644
--- a/tempest/tests/volume/base.py
+++ b/tempest/tests/volume/base.py
@@ -85,7 +85,7 @@
         operate in an isolated tenant container.
         """
         admin_client = cls._get_identity_admin_client()
-        rand_name_root = cls.__name__
+        rand_name_root = rand_name(cls.__name__)
         if cls.isolated_creds:
             # Main user already created. Create the alt one...
             rand_name_root += '-alt'