use prefix.description instead of VRF for identifying subnets in NetBox

[mmguero]

My original understanding of what a VRF is was incorrect, and I now think that using it as the way to tie a name to a Prefix was a mistake.

I think it would be better to use just the ipam/prefixes data model and to use description to store the name rather than do what we're currently doing.

This will inconvenience some existing users as they'll have to go fill in the prefix descriptions. Perhaps on initialization we could do that for them manually, though, if a prefix exists, its description is empty, and it has a VRF associated with it.