WIP: Remove the 'dns' options from libvirt

[eparis]

This will allow the libvirt nodes to use the hypervisor's DNS instead of
having to hard code some upstream DNS server. Previously this was a
problem if your local machine pointed to the libvirt dnsmasq for DNS
resolution of the nodes. Any unknown address would cause an inifnite
loop. Now, the libvirt dnsmasq will respond that the name is unknown
instead of forwarding it to the hypervisor's DNS server.

Since i can't see a reason for the DNS option, I take it out. Now it
'just works'.

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[eparis]

This PR requires dmacvicar/terraform-provider-libvirt#376

[wking]

Looks like it's from coreos/tectonic-installer#3213. It came in with a bunch of other libvirt stuff, and I didn't see any notes motivating this property specifically.

[eparis]

I tried to just rip and out and see what broke. Turns out, DNS query loops are bad.

[openshift-bot]

@eparis: PR needs rebase.

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[wking]

Obsoleted by #214?