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

rpc/dataconn/timeoutconn/internal/wireevaluator/wireevaluator.go

@@ -67,7 +67,7 @@ func main() {
 	case "connect":
 		tc, err := getTestCase(args.testCase)
 		noerror(err)
-		connecter, err := transportconfig.ConnecterFromConfig(global, conf.Connect)
+		connecter, err := transportconfig.ConnecterFromConfig(global, conf.Connect, config.ParseFlagsNone)
 		noerror(err)
 		wire, err := connecter.Connect(ctx)
 		noerror(err)
@@ -75,7 +75,7 @@ func main() {
 	case "serve":
 		tc, err := getTestCase(args.testCase)
 		noerror(err)
-		lf, err := transportconfig.ListenerFactoryFromConfig(global, conf.Serve)
+		lf, err := transportconfig.ListenerFactoryFromConfig(global, conf.Serve, config.ParseFlagsNone)
 		noerror(err)
 		l, err := lf()
 		noerror(err)