Add run_check_index_rst.py and improve documentation

This patch adds `run_checkdocs.py` helper, which checks all
existing files and it's documentation. This check is added in .travis.yml
and will be executed with each test.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>

Moved generic subtest locating / naming code into
dockertest.environment.SubtestDocs.  This module by definition
has no dockertest-dependencies).  Added methods for extracing
docstrings from subtest modules and joining them
all together.  Added unittests to verify all functionality.

Updated Makefile to properly identify changed subtest modules so
subtest documentation will be rebuilt automatically.  Updated index.rst
to include subtests.rst file.  Added subtests.rst as a dependency
to all targets.

Modified travis.yml to build documentation prior to running
documentation checks.  Modified run_checkdocs.py to point
at subtests.rst instead of index.rst

Signed-off-by: Chris Evich <cevich@redhat.com>