subtests...run_memory: Maintain test

*  Fixed missing raise statement for exception
*  Improve exception message w/ actual missing path
*  rewrote read_cgroup() -> cgroup_fullpath()
*  Replace check_cgroup_exist() with just os.path.exists() call.
*  Update all DockerCmd to pass SubSubtest instance.
*  Updated default configuration so test passes.
   (Made note of requested documentation update)

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