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
Merge "Documents keystone v3 API usage - part 1"