dockertest.documentation: Integrate INI parser

*  Class and static method name changed, but interface
   and behavior same.  Update ConfigINIParser.__init__() to use
   SubtestDoc.module_filenames(root_path).
*  Removed unnecessary support for nested keys and hiding
   certain types of format-parsing errors.  Added unittests
   to verify some DocBase class methods & behavior.
*  Pull in default config. section docs by way of prefixing
   subtest list with documentation substitution key.

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