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

transport/tls/connect_tls.go

@@ -18,12 +18,12 @@ type TLSConnecter struct {
 	tlsConfig *tls.Config
 }
 
-func TLSConnecterFromConfig(in *config.TLSConnect) (*TLSConnecter, error) {
+func TLSConnecterFromConfig(in *config.TLSConnect, parseFlags config.ParseFlags) (*TLSConnecter, error) {
 	dialer := net.Dialer{
 		Timeout: in.DialTimeout,
 	}
 
-	if fakeCertificateLoading {
+	if parseFlags&config.ParseFlagsNoCertCheck != 0 {
 		return &TLSConnecter{in.Address, dialer, nil}, nil
 	}