Schedules all system cron jobs.

def ScheduleSystemCronJobs(names=None):
  """Schedules all system cron jobs."""

  errors = []
  disabled_classes = config.CONFIG["Cron.disabled_cron_jobs"]
  for name in disabled_classes:
    try:
      cls = registry.SystemCronJobRegistry.CronJobClassByName(name)
    except ValueError:
      errors.append("Cron job not found: %s." % name)
      continue

  if names is None:
    names = iterkeys(registry.SystemCronJobRegistry.SYSTEM_CRON_REGISTRY)

  for name in names:

    cls = registry.SystemCronJobRegistry.CronJobClassByName(name)

    enabled = cls.enabled and name not in disabled_classes
    system = rdf_cronjobs.CronJobAction.ActionType.SYSTEM_CRON_ACTION
    args = rdf_cronjobs.CronJobAction(
        action_type=system,
        system_cron_action=rdf_cronjobs.SystemCronAction(job_class_name=name))

    job = rdf_cronjobs.CronJob(
        cron_job_id=name,
        args=args,
        enabled=enabled,
        frequency=cls.frequency,
        lifetime=cls.lifetime,
        allow_overruns=cls.allow_overruns)
    data_store.REL_DB.WriteCronJob(job)

  if errors:
    raise ValueError(
        "Error(s) while parsing Cron.disabled_cron_jobs: %s" % errors)