slaps.request: don't fetch schema for frontend software release

The frontend software URL is a virtual URL that does not have a
corresponding schema, so fetching the schema is just a waste of time.

Generally speaking, fetching the schema impact performance a lot, so it
would be nice to cache them. In practice most of the requests made
during instantiation are frontend request, so this is a first easy step
to fix the performance impact of fetching the schema.