Disable filtering for tiles_url

The default ``tiles_url`` parameter in ``config.yaml.in`` is set to:
```
tiles_url:
- http://a.tiles.${vars:host}/${vars:instanceid}/tiles
- http://b.tiles.${vars:host}/${vars:instanceid}/tiles
- http://c.tiles.${vars:host}/${vars:instanceid}/tiles
- http://d.tiles.${vars:host}/${vars:instanceid}/tiles
```
In the ``viewer.js```template, with simply
```
var WMTS_OPTIONS = {
        url: ${tiles_url},
```
we get a syntax error because ``tiles_url`` is rendered as an "escaped" string instead of an array.