Code review comment for lp://qastaging/~jason-koelker/nova/lp819477

The ability to allow for overlaid networks is pretty important especially for a service provider. The current behavior only allows for the addition of a single network block per network for Flat networks. Sometimes we'll need to add several blocks of IPs to a Flat network and with the way the code works currently, it would generate a new VIF per block added. This makes it really hard to scale a zone out. A single flat network can have many overlaid network blocks on it and this patch would allow for that to work correctly.

It also gives us the ability to add smaller blocks at a time and move them around if we need to. The case where this makes sense is if a lot of space gets freed up in a zone while another zone starts nearing IP space capacity.