Refactorings for the list servers tests

* Replaced assertTrue's with assertIn/assertNotIn
* Fixed pep8 issues
* Removed detailed messages for assertIn/NotIn as
  these assertions provide very detailed output

Change-Id: I8fc2d9b86f932cff3bb837a2bd07c63f8f0492c6
Merge "Refactorings for the list servers tests"