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
Merge "Cleans up ServiceUser.__str__ method"