Fix OmnibusHelper#service_{up,down}? implementation to check stdout

OmnibusHelper#service_up? and its down counterpart would only check the return value of sv to determine whether a service was running. However, this is not sufficient because sv will return 0 if all commands have been sent to the process in question, regardless of its state. To fix this, we need to inspect the output to see if it starts with "run: ".

Closes #2100 (closed)