rework app installation - more flexibility on specifying compute env at job submission

[magland]

Now that we've got aws batch working pretty well, I am seeing the advantage of making the processor independent of where it runs. I also think that we want to keep open the possibility of having different processors within an app use different docker images, and run in different compute environments. So this is just to say I'd like to rethink how apps are installed. I think it's limiting to specify information on the compute environment (I mean aws vs slurm vs local) at app installation, but it's better if we have the flexibility to choose this at job submission time.