Adds list users tests in v3

This submission adds test_list_users.py which has
list users with params test cases. Hence made changes
in client methods accordingly.

Change-Id: I264440772c2138ddbac74e68f227c6fb989c7b9f
Merge "Adds list users tests in v3"
Merge branch 'master' of https://github.com/openstack/tempest