Because setting PATH in the container is often container specific and maybe
beyond the scope of the Hudson admin (launcher script is often owned by root),
so it's desirable for us to make PATH configurable for each node.

While we are at it, we might just as well let all environment variables
configurable in general. This could also simplify slave set up.

See http://www.nabble.com/Configuring-PATH-tf4395770.html