The exit status can be propagated from apps by using "systemctl exit"
since systemd v227. rkt checks the systemd version to know whether it
can use "systemctl exit". However, systemd from coreos is back porting
the patches, so we skip the systemd version check on the coreos flavor.

See rkt#1460

Now that the exit code is propagated from the app to rkt, returning 42
from /inspect would make the rkt command fails. The API service tests
don't actually test the exit code, so it is safe to remove that.

This reverts commit e9238f9.

Now that we updated to v225 on flavor coreos, We don't support old
systemd versions so we can actually check for the exit status
everywhere.