Documents keystone v3 API usage - part 1

docs/source/using-api.rst documents how to use the V2 API. This patch
starts the equivalent documentation (using-api-v3.rst) about how to
use Keystone v3 API. Introduction and Authentication sections have
been written.

Change-Id: I5a2d45d80ba712492717a75ac5901f5ca775daa4
Partial-Bug: #1260527
Controllable redirect handling

The requests library handles redirects as a browser would, which has the problem
that forwarding a POST will convert it to a GET. This is not necessarily
intuitive for an API. Handle all redirection manually and provide some
control over how far redirection can go.

Closes-Bug: 1267286
Change-Id: I24596a9c39cc29db28a66c3053ef3954b33abf90
Merge "Controllable redirect handling"
Merge "Documents keystone v3 API usage - part 1"