Ceph » mgr » Dashboard

If you don't set mgr/dashboard/url_prefix, the static resouces are 404 due to the path:

```
<title>Ceph</title>
<meta content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no" name="viewport">
<link rel="stylesheet" href="//static/AdminLTE-2.3.7/bootstrap/css/bootstrap.min.css">
<link rel="stylesheet" href="//static/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<link rel="stylesheet" href="//static/AdminLTE-2.3.7/dist/css/AdminLTE.min.css">
<link rel="stylesheet" href="//static/AdminLTE-2.3.7/dist/css/skins/skin-blue.min.css">
<link rel="stylesheet" href="//static/AdminLTE-2.3.7/plugins/datatables/jquery.dataTables.min.css">
<script src="//static/AdminLTE-2.3.7/plugins/jQuery/jquery-2.2.3.min.js"></script>
<script src="//static/AdminLTE-2.3.7/plugins/sparkline/jquery.sparkline.min.js"></script>
<script src="//static/libs/rivets/0.9.6/rivets.bundled.min.js"></script>
<script src="//static/libs/underscore.js/1.8.3/underscore-min.js"></script>
<script src="//static/AdminLTE-2.3.7/bootstrap/js/bootstrap.min.js"></script>
<script src="//static/AdminLTE-2.3.7/dist/js/app.min.js"></script>
<script src="//static/AdminLTE-2.3.7/plugins/datatables/jquery.dataTables.min.js"></script>
<script src="//static/libs/moment.js/2.17.1/moment.min.js"></script>
<script src="//static/libs/Chart.js/2.4.0/Chart.min.js"></script>
<script>
```

The browser will want to access http://static/

This is a regress casued by https://github.com/ceph/ceph/pull/19758

Tested with ceph master branch and Chromium 62.0.3202.89