Docker: Update dockertest.subtest docs

*  ``index.rst`` now pulls in entire ``subtest`` module
   instead of class-by-class (old problem of ``Subtest``
   class not appearing, doesn't happen now that there is
   ``SubBase``.

*  Updated docstrings on a few undocumented attributes.

*  Added double-backquotes around some symbol names in
   docstrings.

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