Autoload Nova extensions

This patch adds logic to autoload standard novaclient extensions
(e.g. baremetal) and any third-party extensions that might
be installed. Thus, resource plugins that make use of
installed extensions are able to use the standard OpenStackClients
rather than have to a) subclass and override OpenStackClients
or b) create a shadow nova client within the resource plugin
with the desired extension loaded.

Change-Id: Ifdcaa896babda0e1e7e10a55253a8baa37582264
Merge "Autoload Nova extensions"