Make identity providers use uri_join_paths

Instead of defining the same _path function in each of these providers,
I have moved it to uri_join_paths in openstack-common. This change
simply makes use of that new function.

All tests pass.

Change-Id: Ie821d4210f1a34f6aa9f259d8b591f95c6304485
Merge "Make identity providers use uri_join_paths"