Add discovery based on list of subnets

It would be very helpful if we could specify a list of subnets that are continously scanned for new hosts. I know there is a python script for this you can run in the command line, but this should be a persistent list of subnets that grows over time, and usable from GUI instead of CLI, as some deployment types (like kubernetes) do not plan to allow access to the underyling FS/OS anyways.