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

client/migrate.go

@@ -129,7 +129,7 @@ func doMigrateReplicationCursor(ctx context.Context, sc *cli.Subcommand, args []
 	}
 
 	cfg := sc.Config()
-	jobs, err := job.JobsFromConfig(cfg)
+	jobs, err := job.JobsFromConfig(cfg, config.ParseFlagsNone)
 	if err != nil {
 		fmt.Printf("cannot parse config:\n%s\n\n", err)
 		fmt.Printf("NOTE: this migration was released together with a change in job name requirements.\n")