Embed the ProviderClient struct in ServiceClient.

This lets us inherit the AuthenticatedHeaders method, rather than
having to walk the references all the time.
Remove the `.Provider.` bit.
More .Provider references in OS identity v3.
Use the fake ServiceClient from testhelper.
Provider: references in the OpenStack client.
:fire: more manual ServiceClient fixtures.
:fire: :fire: :fire:
Last few references.
Merge pull request #248 from smashwilson/embed-provider

Embed ProviderClient in ServiceClient.