Extend virt.network_define to create NAT networks

[cbosdo]

What does this PR do?

virt.network_define can already define NAT virtual networks by setting the forward attribute to nat... however libvirt fails because it needs an IP configuration in such a case but virt.network_define doesn't have parameters for it.

The PR adds the missing IP configuration to both virt.network_define module and virt.network_running state to allow properly setting up NAT virtual networks.

What issues does this PR fix or reference?

Previous Behavior

Applying the following state fails with XML error: nat forwarding requested, but no IP address provided for network 'net1' error.

net0:
  virt.network_running:
    - name: net0
    - bridge: net0-br
    - forward: nat
    - autostart: True

New Behavior

Changing the state to the following creates a proper network.

net0:
  virt.network_running:
    - name: net0
    - bridge: net0-br
    - forward: nat
    - ipv4_config:
        cidr: 192.168.44.0/24
        dhcp_ranges:
          - start: 192.168.44.10
            end: 192.168.44.25
          - start: 192.168.44.110
            end: 192.168.44.125
    - autostart: True

The dhcp_ranges part is not strictly required for the NAT network creation and are thus optional, but this is so commonly used that I added them in this PR.

Tests written?

Yes

Commits signed with GPG?

Yes

[cbosdo]

May be that PR could also be submitted to 2019.2.1 since this typically a bug fix... even though it adds some parameters to the API.

[xeacott]

Looks wonderful @cbosdo thank you for getting this together. Left a comment that should get the rest of the checks passing!

[cbosdo]

for some weird reason, the tests are still showing as not completed after a few hours...

[waynew]

@cbosdo I think that's due to some of the work we're doing on stabilizing our test suite. You can read https://groups.google.com/forum/#!topic/salt-announce/5RJjtDYszx8 for more info, but I'm pretty sure that we temporarily turned off our PR builds last night, operating under a noisy neighbor sort of theory.

Please be patient while we work out the kinks, but also feel free to ping regularly if you don't see any updates happening!

[cbosdo]

@waynew Thanks for the update on that!

[waynew]

re-run full all

[waynew]

FWIW this looks good to me - I'm not familiar with virt/nat stuff, but I don't see anything otherwise in the code that would concern me!

[cbosdo]

Rebased on master