certs: Implement --skip-renew option
This option is useful primary during upstart when we want to start Sentinel:proxy quickly. There surely must be a valid cert before proxy start so it must be downloaded when there's no. But in case we have a valid cert - which is nearly expired - we do not want ask for renew right on. The renew could be done later on - probably as a backgroud job.
Edited by Vojtech Myslivec