MC-29795: Some environment variables do not lock the settings in the backend

Author: dyushkin

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

@@ -43,6 +43,10 @@ public function render(\Magento\Framework\Data\Form\Element\AbstractElement $ele
             $element->setDisabled(true);
         }
 
+        if ($element->getIsDisableInheritance()) {
+            $element->setReadonly(true);
+        }
+
         $html = '<td class="label"><label for="' .
             $element->getHtmlId() . '"><span' .
             $this->_renderScopeLabel($element) . '>' .

app/code/Magento/Config/Test/Unit/Block/System/Config/Form/FieldTest.php

@@ -7,8 +7,6 @@
 
 /**
  * Test how class render field html element in Stores Configuration
- *
- * @package Magento\Config\Test\Unit\Block\System\Config\Form
  */
 class FieldTest extends \PHPUnit\Framework\TestCase
 {
@@ -72,6 +70,7 @@ protected function setUp()
                 'getCanUseDefaultValue',
                 'setDisabled',
                 'getTooltip',
+                'setReadonly'
             ]
         );
 
@@ -179,7 +178,8 @@ public function testRenderInheritCheckbox()
         $this->_elementMock->expects($this->any())->method('getCanUseWebsiteValue')->will($this->returnValue(true));
         $this->_elementMock->expects($this->any())->method('getCanUseDefaultValue')->will($this->returnValue(true));
         $this->_elementMock->expects($this->once())->method('setDisabled')->with(true);
-        $this->_elementMock->expects($this->once())->method('getIsDisableInheritance')->willReturn(true);
+        $this->_elementMock->method('getIsDisableInheritance')->willReturn(true);
+        $this->_elementMock->method('setReadonly')->with(true);
 
         $expected = '<td class="use-default">';
         $expected .= '<input id="' .