Change GEOSERVER path to be relative

I discovered that layers are stored in GeoNode DB with the source full hostname hardcoded. This is causing troubles when moving an installation under a new hostname (i.e. moving from staging to production):

```sql
oqplatform=# SELECT id, map_id, ows_url FROM maps_maplayer WHERE ows_url IS NOT NULL OR ows_url != '';
 id | map_id |                       ows_url                        
----+--------+------------------------------------------------------
 20 |     30 | https://platform-staging.openquake.org/geoserver/wms
 21 |     30 | https://platform-staging.openquake.org/geoserver/wms
 41 |     38 | https://platform-staging.openquake.org/geoserver/wms
 42 |     38 | https://platform-staging.openquake.org/geoserver/wms
 50 |     39 | https://platform-staging.openquake.org/geoserver/wms
 51 |     39 | https://platform-staging.openquake.org/geoserver/wms
 58 |     40 | https://platform-staging.openquake.org/geoserver/wms
 59 |     40 | https://platform-staging.openquake.org/geoserver/wms
 68 |     41 | https://platform-staging.openquake.org/geoserver/wms
 76 |     42 | https://platform-staging.openquake.org/geoserver/wms
 77 |     42 | https://platform-staging.openquake.org/geoserver/wms
 99 |     50 | https://platform-staging.openquake.org/geoserver/wms
(12 rows)
```

This PR should makes the ```ows_url(s)``` relative (```/geoserver/wms```) and not absolute. Before merging this must be tested intensively in all scenarios (dev, prod, prod+https).
Probably continue to use OGC_SERVER is more flexible and consistent than use GEOSERVER_URL directly.