fix issue on Ubuntu with check_running for courier-imap service

When specifying the os_by_host as Ubuntu, the 'check runnning' of courier-imap service (Ubuntu 12.04 tested) is not working.
indeed, the return of `service courier-imap status` is the following:
 - when running: `* Courier IMAP server is running`
 - when not running: `* Courier IMAP server is not running`