Proposal: replacement of config with env vars

Remove support for the strong-pm configuration file. The features
that were configurable from it are better configured using other
interfaces.

This PR proposes, by way of documentation, to add pmctl sub-commands
for controlling application environment variables and injecting files
into the application at deployment time.

Environment variable examples:
- List current environment:
  - `slc pmctl env`
- Add/set single variable:
  - `slc pmctl env PORT=4321`
- Add/set multiple variables:
  - `slc pmctl env PORT=4321 WORKERS=4 NODE_ENV=staging`
- Unset single variable:
  - `slc pmctl env PORT=`
- Unset multiple variables:
  - `slc pmctl env PORT= WORKERS= NODE_ENV=`
- Corner case, set to empty string:
  - `slc pmctl env PORT=''`
- At service installation time:
  - `slc pm-install --env=.env`

File injection examples:
- upload strongloop.json from current directory and inject it into the
  application environment at deployment time.
  - `slc pmctl inject-file strongloop.json`
- upload the file at `/path/to/global/default` and inject it into the
  applicaiton environment at deployment time as `strongloop.json`
  - `slc pmctl inject-file /path/to/global/default:strongloop.json`
- remove the strongloop.json file stored in strong-pm for deployments
  - `slc pmctl inject-file strongloop.json:`
- remove the strongloop.json file from application environment if it
  is found at deployment time.
  - `slc pmctl inject-file :strongloop.json`
- at service installation time, pre-define a strongloop.json injection:
  - `slc pm-install --inject-file local/path:strongloop.json`