ssl: Set a global boolean to enable SSL across Minio (#3558)

We have been using `isSSL()` everywhere we can set
a global value once and re-use it again.

cmd/lock-rpc-server.go

@@ -289,7 +289,7 @@ func (l *lockServer) lockMaintenance(interval time.Duration) {
 			secretKey:       serverCred.SecretKey,
 			serverAddr:      nlrip.lri.node,
 			serviceEndpoint: nlrip.lri.rpcPath,
-			secureConn:      isSSL(),
+			secureConn:      globalIsSSL,
 			serviceName:     "Dsync",
 		})