No need to use HOSTNAME or fill TERRA_TILES_HOSTNAME by default

[submarcos]

HOSTNAME is used to set TERRA_TILES_HOSTNAME by default.

TERRA_TILES_HOSTNAME is used to propose absolute urls for tile url patterns in tilejson.

It is not required to have a default value for this, using request.build_absolute_uri should be sufficient to propose a default absolute url. If headers are correctly set, tiles url will be generated with original request url (compatible with external frontend url in production). In dev mode, backend url will be used and working with CORS allowed