Actions API: Move OpenStack actions to mistral-extra repo

* Need to separate OpenStack actions and move them into a separate repo with the ability to install only part of them during installation time

Notes from the Austin Summit discussions:

Mistral contains the ability to dynamically generate OpenStack actions using OpenStack clients:
    http://git.openstack.org/cgit/openstack/mistral/tree/mistral/actions/openstack/actions.py
    It would be useful to consider splitting this out into a separate project. Benefits would be that Mistral proper no longer depends on all the OpenStack clients, and users wishing to create custom Mistral actions would have a more formalized interface to extend. This would allow projects like TripleO to write custom Mistral actions that can more easily access auth credentials within the workflow (like the OpenStack actions do). This follows a similar approach to Heat in that a user extending a Heat resource wouldn't have to re-create their own auth mechanisms like you do today with Mistral (unless you are willing to use internal Mistral context classes, etc.). Eventually it might even be possible to share some of the client creation/auth logic with a project like Heat as well....(shardy) - +1 heat has very similar requirements here, and we've got a client plugin interface that is made available to the resource plugins (similar model to mistral action plugins), it does seem possible we could split out that code from heat and potentially reuse it https://github.com/openstack/heat/tree/master/heat/engine/clients