Cleanup of identity/admin/test_users.py

Cleanup of test_users.py, partially to help debug 1103081 where
test_delete_user sometimes fails. This is not a fix for the bug,
but the extra assertion will help narrow down where the problem
is if it occurs in the future

* Add status code assertions to more thoroughly narrow down where
  errors occur

* Replace startswith type status code assertions to check for the
  actual expected status codes on success rather than just 2*

Change-Id: I57e560c6ce85743c09380fa93d7c752ecf087b75
Merge "Cleanup of identity/admin/test_users.py"