etcdserver: add 'ClientCertAuthEnabled' option

etcdserver/config.go

@@ -56,6 +56,9 @@ type ServerConfig struct {
 	StrictReconfigCheck bool
 
 	EnablePprof bool
+
+	// ClientCertAuthEnabled is true when cert has been signed by the client CA.
+	ClientCertAuthEnabled bool
 }
 
 // VerifyBootstrap sanity-checks the initial config for bootstrap case