rest-server: option to recreate shell login environment in job-submit API

[tpatki]

Follow-up from PR #7: current cwd/environment defaults are minimal
and won't replicate elaborate per-cluster setups (modules, profile
scripts, etc).
@garlick's idea: opt-in flag to spawn a real login shell as the
user and use its environment instead. (Revisit once we have the simple REST APIs working.)

[garlick]

Maybe we should just keep this idea in our back pocket in case it turns out to be a practical necessity. Encouraging batch scripts to be self-contained and load the exact environment-modules they need is probably not a bad thing.