Resin-Supervisor: test its API endpoints

[pcarranzav]

For each of the supervisor's API endpoints (docs) hit the endpoint and check response and expected behavior. (Shutdown might be hard to test, but it should be okay if reboot works).

Please ping me for any questions about the expected behavior of the endpoints.

[pcarranzav]

I recommend building and pushing a test application that runs and hits the endpoints logging results. We can work on it together if you want.