WIP: add persisted environment variables

 * [x] Create persisted environment on pm start
 * [x] Inject persisted environment into `prepare` and `run` phases
 * [x] Set environment variables via `pmctl env-set K=V [K2=V2...]`
 * [ ] CLI argument parsing/validation
 * [ ] Document CLI

Additional functionality for later PR:
 * Get environment variables via `pmctl env-get [KEYS...]`
 * Expose environment via REST API

Refs: strongloop-internal/scrum-nodeops#99