Add edxapp_custom_debian_pkgs field to install extra packages

The idea of this PR: making opportunity to install some extra debian packages that your python packages might require, for example.

You can use `edxapp_custom_debian_pkgs` key in `server-vars` to list packages you need.

This is not supposed to be mirged into master yet. It also might be closed. I wanted just to give an idea what I, as a edx-platform developer, lack of. :)