Pin environments to certain platform releases

right now, afaik we only have one global "Leapfile" where you can specify the "@platform_directory_path".

But with the bitmask provider envs, we would need a different platform directory for the demo env (checkout of 0.6.0) and the dev/unstable env (checkout of develop branch).

Maybe i'm missing sth though.

(from redmine: created on 2014-12-24, closed on 2015-02-03)