Migrate image client group to client factory

Migrate the service clients in the image group to the new client
factory. The client factory for image was already available;
this change simply makes the client attributes in clients
use the factory rather than instantiating new clients.

With this approach the attributes (e.g. image_client etc) stay
on the tempest side. I don't want to make the existing names a stable
interface.

Setting the client attributes in _set_image_clients kills the lazy
loading provided by the client factory, but it's an intermediate step
towards it. Next patch will move to actual lazy loading of image
clients.

Partially-implements: bp client-manager-refactor
Change-Id: I6c3d604e63dbe1cbe30a4bac333f1b8d471ee543
1 file changed