MAGETWO-64523: Add static test on forbidden "final" keyword and eliminate it usage in code

 - implemented PHPMD rule for detecting final classes and methods
 - created PHPMD ruleset for Magento specific design rules

Author: Volodymyr Kublytskyi

dev/tests/static/framework/Magento/MessDetector/resources/rulesets/design.xml

@@ -0,0 +1,30 @@
+<?xml version="1.0"?>
+<ruleset name="Magento Specific Design Rules"
+         xmlns="http://pmd.sf.net/ruleset/1.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://pmd.sf.net/ruleset/1.0.0 http://pmd.sf.net/ruleset_xml_schema.xsd"
+         xsi:noNamespaceSchemaLocation="http://pmd.sf.net/ruleset_xml_schema.xsd">
+    <rule name="FinalImplementation"
+          class="Magento\MessDetector\Rule\Design\FinalImplementation"
+          message= "The {0} {1} declared as final.">
+        <description>
+            <![CDATA[
+Final keyword is prohibited in Magento as this decreases extensibility and customizability.
+Final classes and method are not compatible with plugins and proxies.
+            ]]>
+        </description>
+        <priority>1</priority>
+        <properties />
+        <example>
+            <![CDATA[
+final class Foo
+{
+    public function bar() {}
+}
+class Baz {
+    final public function bad() {}
+}
+            ]]>
+        </example>
+    </rule>
+</ruleset>

dev/tests/static/framework/Magento/MessDetector/src/Rule/Design/FinalImplementation.php

@@ -0,0 +1,30 @@
+<?php
+/**
+ * Copyright © 2013-2017 Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\MessDetector\Rule\Design;
+
+use PHPMD\AbstractNode;
+use PHPMD\AbstractRule;
+use PHPMD\Rule\ClassAware;
+use PHPMD\Rule\MethodAware;
+
+/**
+ * Magento is a highly extensible and customizable platform.
+ * Usage of final classes and methods is prohibited.
+ */
+class FinalImplementation extends AbstractRule implements ClassAware, MethodAware
+{
+
+    /**
+     * @inheritdoc
+     */
+    public function apply(AbstractNode $node)
+    {
+        if ($node->isFinal()) {
+            $this->addViolation($node, [$node->getType(), $node->getFullQualifiedName()]);
+        }
+    }
+}