Update Tox JJB Command

Update Tox JJB Command

This allows using the command to only test one project's changes. For
example:

  tox -e jjb -- jjb/releng

Will only test jobs under 'jjb/releng'. If jobs have dependencies, of
multiple project jobs need to be tested, they can be included using the
jjb ':' syntax, as anything after '--' is passed directly to jjb:

  tox -e jjb -- jjb/doctor:jjb/functest

Change-Id: I4b71803a1d6ed4f62d50cdca87183a68b20fae5a
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>