dockertest: Doc. bug.

Clarify ``stuff`` and ``sub_stuff`` as dictionary-types, initialized
empty.  Using them as another type, requires re-assigning during
``initialize()`` since static assignment would otherwise get clobbered
in the constructor.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #187 from cevich/no_clobber_stuff

dockertest: Doc. bug.