Docker: Allow disable individual sub-subtest

*  Changed config get_boolean() method to
   examine very specific/defined set of strings
   for conversion.

*  Only log subtest configuration options
   if there will be no sub-subtests running.
   Subsubtests inherit the subtest configuration
   and if need to examine separately, the
   keyval log shows them apart.

*  Added DockerSubSubtestNAError exception
   specific to sub-subtests.

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