[Framework][Workflow] Added support for interfaces

Author: vudaltsov

DependencyInjection/Configuration.php

@@ -329,8 +329,8 @@ private function addWorkflowSection(ArrayNodeDefinition $rootNode)
                                         ->prototype('scalar')
                                             ->cannotBeEmpty()
                                             ->validate()
-                                                ->ifTrue(function ($v) { return !class_exists($v); })
-                                                ->thenInvalid('The supported class %s does not exist.')
+                                                ->ifTrue(function ($v) { return !class_exists($v) && !interface_exists($v); })
+                                                ->thenInvalid('The supported class or interface "%s" does not exist.')
                                             ->end()
                                         ->end()
                                     ->end()