Docker: Segment subtest Init, add enable option

*  Broke down SubTest __init__ into individual
   private methods for each behavior segment
   to make it easier to read/debug.

*  Added ``enable`` configuration (on by
   option ("True" by default) to default
   config section.  Allows individual
   tests to be marked for skipping
   with or without using ``--args``
   csv list.

*  Updated docs.

*  Bumped revision version to 0.5.3 (no interface
   changes)

Signed-off-by: Chris Evich <cevich@redhat.com>