DNS not configured on head node if set previously in /etc/network/interfaces

How to reproduce:
1) Install an head node with Ubuntu 14.04
2) Configure the network and setup DNS using dns-nameservers under any interface that is not the loopback interface (example below)

auto em1
iface em1
address 192.168.1.1
netmask 255.255.255.0
gateway 192.168.1.254
dns-nameservers 8.8.8.8

3) Deploy CORD on the head node

As result, the interface config is overwritten during the CORD installation.

The only ways to make the deployment work right away are:
1) put the dns-nameservers line under the localhost interface
2) put nameserver entry in /etc/resolv.conf

The problem has been never seen before because
1) developers usually install manually CiaB, which sets up DNSs in /etc/resolv.conf (task in platform-install)
2) Physical PODs are usually deployed through automation, which uses an external MAAS to install ubuntu on the head node. As part of that, the 8.8.8.8 DNS address is added under the loopback interface.