Description of problem:
There is lack of support for managing network subnets in oVirt UI.

My suggestion is that for networks of type external, we should expose a tab for subnets, like we do with vNIC profiles. Subnets should be entities that can be viewed, created, edited or removed easily.

Reasons:
1. One can imagine a scenario where CIDR, DNS or gateway of existing subnet needs to be changed. The workaround is to create a new network with subnet with the required changes.
2. There is no way to view the list of existing network subnets.
3. There is no way to delete subnets. We always keep the subnets in the provider DB, even after removing a network which have subnet attached.


Version-Release number of selected component (if applicable):
oVirt Engine Version: 4.1.0-0.0.master.20161120231325.gitba51756.el7.centos