MAGETWO-46533: [Github] MAGE_MODE is changed back to default if database config is changed with setup:config:set

- avoiding overwritting MAGE_MODE, if already set in config.

Author: mazhalai

setup/src/Magento/Setup/Model/ConfigGenerator.php

@@ -235,7 +235,9 @@ public function createXFrameConfig()
     public function createModeConfig()
     {
         $configData = new ConfigData(ConfigFilePool::APP_ENV);
-        $configData->set(State::PARAM_MODE, State::MODE_DEFAULT);
+        if ($this->deploymentConfig->get(State::PARAM_MODE) === null) {
+            $configData->set(State::PARAM_MODE, State::MODE_DEFAULT);
+        }
         return $configData;
     }