Fix a slew of issues related to deletion of images and containers. Also
added --assume_yes to be consistent with other commands. This will benefit
dbus interaction as well given that we do not want confirmation occuring
over dbus.
Issues that were fixed:
* Proper catch and error for trying to delete all images|containers when no images|containers exist
* Catch and error when no image|container is given by user and no --all
* when --storage and --all are used, only images|containers in that backend are now deleted
* when --all is given but not --storage, all images|containers from all backends are now deleted.
* Fixed logical error when deleting containers
* Deletion of images|containers now is consistent in its confirm and abort messages.
Added unittests.
Closes: #864
Approved by: rhatdan
