MDL-45113 auth: add is_configured method and convert auth_db to use it

lib/authlib.php

@@ -212,6 +212,15 @@ class auth_plugin_base {
         return true;
     }
 
+    /**
+     * Returns false if this plugin is enabled but not configured.
+     *
+     * @return bool
+     */
+    public function is_configured() {
+        return false;
+    }
+
     /**
      * Indicates if password hashes should be stored in local moodle database.
      * @return bool true means md5 password hash stored in user table, false means flag 'not_cached' stored there instead