Allow plugins to be specified as a cloud_backend

[andrew plunk]

Right now because clients.py configures the client class to use when it's module is imported. This is a problem if you want to use any of your plugins as a cloud_backend. clients.py should be changes so that it does not choose the client class until after the plugins are initialized.

[andrew plunk (andrew-plunk)]

This bug started as a blueprint: https://blueprints.launchpad.net/heat/+spec/cloud-backend-plugins

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/53013

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/53013
Committed: http://github.com/openstack/heat/commit/8ce5e0f88edecd3ab1d58136ebfac7543938d80f
Submitter: Jenkins
Branch: master

commit 8ce5e0f88edecd3ab1d58136ebfac7543938d80f
Author: Andrew Plunk <email address hidden>
Date: Mon Oct 21 16:12:54 2013 -0700

    Allow plugins to be specified as a cloud_backend

    Currently, the cloud_backend configuration option can only use modules
    imported in-tree before clients.py is imported. This does not work for
    plugins because they are dynamically imported in
    heat/engine/resources/__init__.py after clients.py is imported. Change
    the cloud_backend module to be specified when the client class needs
    to be initialized rather than when clients.py is imported. This change
    requires a class to be specified for the cloud_backend rather than a
    module name.

    Change-Id: I8f2b93bfb6a9585d1e7dc52d70092105b6e1bfaa
    Closes-Bug: 1242870