core: Update certificates as late as possible

  This changes aims minimize the unecessary accessed to master while the
  instance is going to be skipped to process or won't post informations to master.

  This in best cases reduces access from 1+n to 1 while n = number of used partitions.
1 job for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Rafael

01:04:07