Cleans up ServiceUser.__str__ method

Reasons:
- ServiceUser.__str__ in trove/tests/util/users.py contains code
  after the return statement.
- This code after unconditional return statement has no use.
- Thus, ServiceUser.__str__ method needs cleanup.

Changes:
- Removes the statements after return statement.
- Adds tenant name to the __str__.

Change-Id: Ic5e35969f435e01d94da7eb434cd8de19e2c9873
Closes-Bug: #1269437

trove/tests/util/users.py

@@ -67,13 +67,9 @@ class ServiceUser(object):
             raise ValueError("'is_admin' must be specified for a user.")
 
     def __str__(self):
-        return "{ user_name=%s, tenant_id=%s, reqs=%s, tests=%d }" % (
-            self.auth_user, self.tenant_id, self.requirements, self.test_count)
-        self.auth_key = auth_key
-        self.tenant = tenant
-        self.tenant_id = tenant_id
-        self.requirements = requirements
-        self.test_count = 0
+        return ("{ user_name=%s, tenant=%s, tenant_id=%s, reqs=%s, tests=%d }"
+                % (self.auth_user, self.tenant, self.tenant_id,
+                   self.requirements, self.test_count))
 
 
 class Users(object):