MXS-2483: Take SSLProvider into use

Servers and listeners now have a SSLProvider member variable that is used
for all SSL related tasks.

server/modules/authenticator/MariaDBBackendAuth/mysql_backend_auth.cc

@@ -140,7 +140,7 @@ static int auth_backend_authenticate(DCB* dcb)
  */
 static bool auth_backend_ssl(DCB* dcb)
 {
-    return dcb->server->ssl_context() != NULL;
+    return dcb->server->ssl().context() != NULL;
 }
 
 extern "C"