Fix OmnibusHelper#service_{up,down}? implementation

sv only returns non-zero status codes if the base name of the application is something other than sv (as discussed in http://supervision.skarnet.narkive.com/KbZ5zFls/bug-for-sv-status-in-lsb-mode#post6).

Using gitlab-ctl will make the program return the right status value.

Closes #2100 (closed)