app: implement functional tests for mutable pods

[lucab]

We currently have no functional tests at all concerning mutable pods features.
This is a request to implement the first batch of tests which will:

• smoke test for rkt app
• check that rkt app is gated behind an env var
• check that app sandbox brings up an empty pod
• check that add/start work
• check that stop/rm work
• check that a mutable pod is still running even without any apps, and that it can be stopped via rkt stop

[lucab]

#3371 addressed most of it. Further checks will come via integration testing from the rktlet.