Support folders and bundled templates for sources

[cbednarski]

Support a folder in place of a source.py file, for both built-in and external sources. So for example:

haproxy
├── haproxy.conf
├── init.d.sh
└── install.py

This will make it easier to include additional files like configs, init scripts, etc., and also make it possible to use ifs to install / configure userland software. Config templates should use jinja syntax.