Figure out how to configure both v4 and v6 DNS servers in dual stack

This is a followup to https://bugzilla.redhat.com/show_bug.cgi?id=2020780 because it appears as though everything is working correctly, but we need to figure out how to make this work within the restrictions of such an environment. To sum up the situation:

• Configuring a DNS server that matches the provisioning network IP version works largely as expected. The one gotcha is that if the provisioning network is IPv6, IPv4 on the provisioning interface must be disabled or it will prevent the interface from coming up completely and the DNS settings won't be applied. This should be documented.
• Configuring a DNS server that doesn't match the provisioning network or in a cluster that doesn't use a provisioning network is problematic. In this case, the provisioning nic cannot be used to store the DNS settings. Since the only other interface we can rely on having is the bridged external nic, we need to store the DNS settings there.

Unfortunately, because that interface is configured with DNS we need to turn off auto-dns on the bridge, and that means we need to represent the bridge configuration in nmstate. This is problematic because we don't want both nmstate and configure-ovs to be managing that interface.

At this point I only see two options:

1. Document the limitation and don't allow custom DNS servers to be configured if they can't be stored on the provisioning nic.
2. Require that the day 1 nmstate config be used to create the OVNK bridge so that it can be managed by only nmstate. This will require testing because I'm not sure configure-ovs will allow this today.

I'm open to other suggestions though.