MQE-541: changed to use stepKey for test step sequencing.

Author: jilu1

src/Magento/FunctionalTestingFramework/Config/Dom.php

@@ -224,7 +224,9 @@ protected function getNodePathByParent(\DOMElement $node, $parentPath)
                 if ($value = $node->getAttribute($idAttributeValue)) {
                     $path .= "[@{$idAttributeValue}='{$value}']";
                     break;
+                    // TODO Remove the following elseif block once the 'mergeKey' attribute has been fully deprecated.
                 } elseif (empty($value) && $idAttributeValue === GlobalConstants::TEST_ID_ATTRIBUTE) {
+                    print "use of the 'mergeKey' attribute will be deprecated in the next release.\n";
                     $idAttributeValue = GlobalConstants::DEPRECATED_TEST_ID_ATTRIBUTE;
                     if ($value = $node->getAttribute($idAttributeValue)) {
                         $path .= "[@{$idAttributeValue}='{$value}']";

src/Magento/FunctionalTestingFramework/Test/Config/Converter/Dom/Flat.php

@@ -86,6 +86,8 @@ public function convertXml(\DOMNode $source, $basePath = '')
                 }
 
                 $nodeData = $this->convertXml($node, $nodePath);
+
+                // TODO Remove the following if block once the 'mergeKey' attribute has been fully deprecated.
                 if (!isset($nodeData[GlobalConstants::TEST_ID_ATTRIBUTE])
                         && isset($nodeData[GlobalConstants::DEPRECATED_TEST_ID_ATTRIBUTE])) {
                     $nodeData[GlobalConstants::TEST_ID_ATTRIBUTE] =