Add support for "CNAME" like custom dns records

[nvgoldin]

Currently we can add to a bridge-nat network:

...
  dns_records:
     custom_name: IP
...

It would be useful to put the VM name there instead of an IP. Implementation wise - this is not supported in Libvirt, but we could deduce the IP in a latter stage, after we allocated IP addresses to the domain. And then use the normal Libvirt hostname aliases.