OSM Tiles should rely on round-robin load balancer, not hardcode subdomains

https://github.com/darribas/contextily/blob/master/contextily/tile_providers.py#L19-L22 specifies `OSM_A `to `OSM_C `with `a` to `b` as (sub-)subdomains. Please always just use the `tile.openstreetmap.org` as any request to it will automatically be shared between the sub-hosts.

Start with contextily/tile_providers.py at lines 19-22, where the OSM tile subdomains are specified. Check how the provider is configured and update it to use tile.openstreetmap.org as requested. Done means the hardcoded OSM subdomains are no longer used for requests.

Written by the indexing model from the issue text.