fixed attribute key in manager node (#692)

Author: einorler

DependencyInjection/Configuration.php

@@ -154,6 +154,7 @@ private function getManagersNode()
         $node
             ->isRequired()
             ->requiresAtLeastOneElement()
+            ->useAttributeAsKey('name')
             ->info('Maps managers to connections and bundles')
             ->prototype('array')
                 ->children()