subnet field requires no host bit

[Joshua Powers]

Summary:
When setting up the networking it is unclear what is meant by "host bits set" if a user enters "192.168.1.100/24" into the subnet field. I believe that because the subnet is asked BEFORE the address and most users are expecting to enter in the address first, that this leads to confusion.

May make sense to then switch these two fields:

Address: 192.168.1.100
Subnet: 192.168.1.0/24

Expected Result:
Better error message than host bits

Actual Result:
"has host bits set" error message

Steps to Reproduce:
1. Download live ISO from 2018-07-11 md5sum: 9e2a51144591db71afd21e6c50d8182f
2. qemu-img create -f raw target.img 10G
3. qemu-img create -f raw target2.img 10G
4. kvm -m 1024 -boot d -cdrom bionic-live-server-amd64.iso -hda target.img -hdb target2.img -serial stdio
5. Accept all defaults, setup custom ipv4 network with static address

[Michael Hudson-Doyle (mwhudson)]

Yeah, this is kinda daft, but it was the same in the bionic GA release. We should fix it soon though.