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
Refactor random url generation into its own method

Previously, endpoint URL were generated randomly before sending endpoint
requests to the Keystone server. Now that Keystone is validating input
on incoming requests, URLs have to match a regular expression in order
to be considered valid. This change makes the endpoint URLs compliant
for the endpoint request.

Change-Id: I365e651632fc4952a483655e51f3958598e059dd
Merge "Refactor random url generation into its own method"
Merge "Adds list users tests in v3"