allow NetBox autopopulation of public IPs if they're explicitly specified

[mmguero]

Working with a government entity that owns several public IP blocks. They want to use these public IP addresses for autopopulation, while our code for that only allows for autopopulating private IP space.

My thought is if they explicitly put the subnets in NETBOX_AUTO_POPULATE_SUBNETS (and/or?) define the network prefixes in netbox then we allow that autopopulation to happen.