rename tests -> api

Now that all the other tests are moved out of the tests directory
we can rename tests -> api to reflect that these tests are api
testing, and need to use only the internal clients.

Clean up references from other parts of OpenStack to the new api
namespace.

Reorder the imports as required with this naming change.

Added README.rst

Change-Id: I19203957f917b59e7c8a3838c590937752461a2f