add boostrap submodule

If you like..

this adds a reference to the stable branch salt-bootstrap in ``saltcloud/deplo/bootstrap``.  On a fresh clone use ``git clone --recursive`` to also obtain bootstrap. On an existing clone you should be able to run ``git submodule init; git submodule update`` to bring this in.

Note that submodules point to a particular commit, so when salt-bootstrap has a new stable version available, the submodule reference needs to be updated with ``git submodule update`` and committed to refer to the new version. 

It should not be necessary to update or deal with submodules otherwise.