Env per proposal

[mhofman]

It would be great to have a way to wire some proposal specific env file sourced for every step.

To make this useful, some environment variables would need to be standardized and set first, e.g. proposal name, step (prepare, execute, use, test), etc.

The motivation is being able to set a SLOGFILE env to capture the outcome of different steps, in particular the execute upgrade step.