Allow to configure DNS for libvirt network.
Allow to configure DNS for libvirt network.

There are needs when special DNS options have to be configured
on a per-network basics.
This change allows to specify domain for a network as well as
DNS SRV records.

Example of network configuration:
  domain: "testing.local"
  dns:
    srvs:
      - name: "sip-server"
        protocol: tcp
        domain: "testing.local"
        port: 1234

Change-Id: I3a59fb31536e63b8dea49642766120c3e7cb11ca

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared