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.

Hmmm...whenever these come up, it signals to me some kind of hierarchy or class-tree structure problem.  I think I see why separating this is needed, but I'd like to ponder if there are implications we may have to fight down the road.

Let me make sure I understand, this is needed so (e.g.) ``images`` can import the base-class, and something else can import both ``images`` and ``subtests`` modules w/o causing circular dep. on ``Subtest`` (and child) classes.  Correct?

Ahh, okay, the issue comes down to treating the "subtest" concept as high-level, but needing to use the base-class at low level, okay, that's where the loop happens.  MMMmm, I smell future-trouble here, will need to think on it more...