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.

in fact, this here is perfectly fine as well, if you want to roll this part along w/ all the test changes into it's own PR, it could probably just be applied, no?  That would save a lot of efforts on cycling with rebasing this (#352).