Acknowledge alerts

If this option is selected, each alert have to be acknowledged by a SysOrb user before the associated check/node is considered in a good state again. This is useful for ensuring that all alerts is handled by a user. It is especially useful for LogChecks where a bad log entry line should always signal a bad log, ie. a good log entry line read sometime after the bad one should not signal that the LogCheck is in a good state again.