Add interface for user backends which are responsible for a specific domain

refs #2153

library/Icinga/Authentication/User/DomainAwareInterface.php

@@ -0,0 +1,17 @@
+<?php
+/* Icinga Web 2 | (c) 2017 Icinga Development Team | GPLv2+ */
+
+namespace Icinga\Authentication\User;
+
+/**
+ * Interface for user backends that are responsible for a specific domain
+ */
+interface DomainAwareInterface
+{
+    /**
+     * Get the domain the backend is responsible for
+     *
+     * @return  string
+     */
+    public function getDomain();
+}