Add option for setting environment during service installation

 * [x] Add `--env ENV` option to set initial environment for deployed application
 * [x] Update pm-install usage output

Based off of #47 
For strongloop-internal/scrum-nodeops#99
Fixing the ellipsis and adding details about the quotation requirements. The reason I want to use a single argument is so that users can do `--set-env $(cat .env)` or similar if they already have their environment in a file. So if I'm sticking to a single argument format, which I am, I choose that one.