Adding environment variable support for jobs

Second pass at environment variables, now no longer limited to Docker tasks.  

Ping @brndnmtthws 
With this, the job-defined env vars will always take precedence.  I think it should be the other way.  Things like `CHRONOS_JOB_NAME` should not be over-ridable.

You could either a) set the variables below before the ones above are set, or b) check if the vars are already set before trying to set them.