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"