Originally we required `role` when switching `shard`'s because I felt it
made it less confusing. However now that we're exploring some more
multi-tenancy work I agree we need to move this condition. Otherwise if
you're using one middleware to switch roles and another to switch
shards, you may be forced to pass the role around in places you're only
concerned with shard. This also simplifies the call for applications
that don't use roles and only have writer shards.