refactor the Manager API.

This changes allows us to have a better distinction between templates &
processes:

- templates are process templates used to launch processes
- process are objects around a real OS process.

Also each templates are owned by an application. An application is a set of
processes templates. I will make easier the association with procfiles and the
HTTP api. Due to that change each process by defaults are associated to a
"system" application.

While I was here I considerably simplified the manager api. All groups*
methods have been removed and new walk* methods have been added to walk in
applications or templtaes. ttin/ttou methods have been replaced by the scale
method as well.