Job, possibility to update env vars

[albertz]

As it was discussed in #178.

Fix #178

[critias]

See my comment on #178

I think putenv should be named set_env and I would avoid creating and compiling an extra file here and just update directly from _sis_environ_updates.

[critias]

It bothers me to introduce a new structure which changes the environment. How about always creating the EnvironmentModifier, but only clean the environment if gs.CLEANUP_ENVIRONMENT is True and set_env would just pass the envs to EnvironmentModifier.set.

That way you should be able to accomplish the same thing without updating the environment in two different places.

[albertz]

I really don't want the string.Template.substitute logic though, so in any case, I need another flag to enable or disable that, maybe per key, or globally. I think per key might be better, because I don't want to break any existing setups, or introduce the situation that you cannot mix it, thus have it incompatible with some setups where you both need this for some env vars, and where you don't want this for other env vars.

[critias]

Let's move the discussion to #178 to avoid two parallel discussions about the same thing.