Resin Supervisor - Adding deltas to a running supervisor

[lifeeth]

Test Steps:

1. Add an application-wide RESIN_SUPERVISOR_DELTA = 1 config var. (It is important that the value is 1).
2. Push an update to the application (e.g. changing what is outputted to the console).

Expected result:

1. When setting RESIN_SUPERVISOR_DELTA, the device should not restart the app.
2. The device should download the update correctly after pushing.
3. Fetching the devices' environment variables should show that "RESIN_SUPERVISOR_DELTA" environment variable is set to 1.