Fix password not strong enough for identity test_groups

Use rand_password method to create users with more complex passwords.
This part was missed in commit I57649f5aac9b1abe1a9961d4b35479372ebee519

Change-Id: I9b06e4f1f026ab6795263969aa10e4ddfc5bae92
Related-Bug: 1448217
diff --git a/tempest/api/identity/admin/v3/test_groups.py b/tempest/api/identity/admin/v3/test_groups.py
index e022023..21fc62a 100644
--- a/tempest/api/identity/admin/v3/test_groups.py
+++ b/tempest/api/identity/admin/v3/test_groups.py
@@ -86,8 +86,7 @@
     def test_list_user_groups(self):
         # create a user
         user = self.client.create_user(
-            data_utils.rand_name('User'),
-            password=data_utils.rand_name('Pass'))['user']
+            data_utils.rand_name('User'), data_utils.rand_password())['user']
         self.addCleanup(self.client.delete_user, user['id'])
         # create two groups, and add user into them
         groups = []