internal: use persisted environment for --metrics

Move the STRONGLOOP_METRICS environment variable out of the Upstart job
and into the initial persisted environment.

@sam-github As discussed in this morning's standup.
/cc @kraman @chandadharap 
thinking about it... probably because pmEnv isn't parsed yet, is it... if set-env could be provided multiple times, and did the parse and assign into a json object, these two options could be more unified, -m would just be a short form of -e, that knew the env var name