Add --skip-cert-check flag to zrepl configcheck to prevent checking cert files

It may be desirable to check that a config is valid without checking for
the existence of certificate files (e.g. when validating a config inside
a sandbox without access to the cert files).

This will be very useful for NixOS so that we can check the config file
at nix-build time (e.g. potentially without proper permissions to read cert
files for a TLS connection).

fixes https://github.com/zrepl/zrepl/issues/467
closes https://github.com/zrepl/zrepl/pull/587

daemon/daemon.go

@@ -51,7 +51,7 @@ func Run(ctx context.Context, conf *config.Config) error {
 	}
 	outlets.Add(newPrometheusLogOutlet(), logger.Debug)
 
-	confJobs, err := job.JobsFromConfig(conf)
+	confJobs, err := job.JobsFromConfig(conf, config.ParseFlagsNone)
 	if err != nil {
 		return errors.Wrap(err, "cannot build jobs from config")
 	}