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

config/config.go

@@ -17,6 +17,13 @@ import (
 	zfsprop "github.com/zrepl/zrepl/zfs/property"
 )
 
+type ParseFlags uint
+
+const (
+	ParseFlagsNone        ParseFlags = 0
+	ParseFlagsNoCertCheck ParseFlags = 1 << iota
+)
+
 type Config struct {
 	Jobs   []JobEnum `yaml:"jobs"`
 	Global *Global   `yaml:"global,optional,fromdefaults"`