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/storage-rpc-client.go

@@ -121,7 +121,7 @@ func newStorageRPC(ep *url.URL) (StorageAPI, error) {
 			secretKey:        secretKey,
 			serverAddr:       rpcAddr,
 			serviceEndpoint:  rpcPath,
-			secureConn:       isSSL(),
+			secureConn:       globalIsSSL,
 			serviceName:      "Storage",
 			disableReconnect: true,
 		}),