Backport for Magento 2.2 - Fixes variables in configuration not being…

… replaced with actual values in the backend form fields.

(cherry picked from commit a8f1729)

app/code/Magento/Config/Block/System/Config/Form.php

@@ -423,6 +423,10 @@ private function getFieldData(\Magento\Config\Model\Config\Structure\Element\Fie
                 $backendModel = $field->getBackendModel();
                 // Backend models which implement ProcessorInterface are processed by ScopeConfigInterface
                 if (!$backendModel instanceof ProcessorInterface) {
+                    if (array_key_exists($path, $this->_configData)) {
+                        $data = $this->_configData[$path];
+                    }
+
                     $backendModel->setPath($path)
                         ->setValue($data)
                         ->setWebsite($this->getWebsiteCode())