Simplifies docker interfaces to minimum adds dockercmd interface ability fix some minor problems in tests

There shouldn't be any conflict with changes in docker documentation. 

Simplifies docker interface mechanism from object composition to
module global variable assignation.
Adds variables in defaults.ini to set default test interface.

   default_container_iface = cli
   default_images_iface = cli
   default_dockercmd_iface = cli

example how to change default interface in whole test:
module global variable DockerImages

images.DockerImage = new interface class.

``remove_after_test = yes`` is the default, so this change can just be discarded (unless I'm missing something)