Support explicit authorization

[Brian Curtin]

We currently take credentials in the Connection but don't do anything with them until you try to do anything. We should probably support something like an `authorize` method on Connection that either succeeds or fails there, instead of applications breaking down at potentially odd spots for somewhat unrelated reasons (think of a create_container failing just for bad credentials, the failure isn't *really* tied to create_container)

This wouldn't be required -- we'd still go grab a token on the first request if you're not already authorized -- but it can be nice to know even earlier than your first call whether or not the app is going to work.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/242681
Committed: https://git.openstack.org/cgit/openstack/python-openstacksdk/commit/?id=0ed87c2cc17d24e123864a58bae2841ca49ce155
Submitter: Jenkins
Branch: master

commit 0ed87c2cc17d24e123864a58bae2841ca49ce155
Author: Brian Curtin <email address hidden>
Date: Fri Nov 6 16:49:43 2015 -0600

    Add authorize method to Connection

    The ability to know your application is going to fail due to a
    credential issue *before* getting into the application and attempting to
    make some API call can be useful. This change adds Connection.authorize,
    which just does the same thing we do implicitly. It's entirely optional
    and is documented as such.

    Co-Authored-By: Qiming Teng (<email address hidden>)
    Closes-Bug: #1513200
    Change-Id: If86f308da909c4ff31f37294a000c6972c947182