Do not produce a broken /e/n/i file.
Do not produce a broken /e/n/i file.

Ifupdown, the package that interprets /etc/network/interfaces when
'service networking restart' is issued, will raise an error if the
same interface is defined twice. So we need to be sure we're not
accidentally overwriting any existing interfaces.

Change-Id: Ibb639999e64292141dcb8eb02820470a246be27c

radez/tripleo-image-elements,qwefi/tripleo-image-elements,qwefi/tripleo-image-elements,agroup/tripleo-puppet-elements,openstack/tripleo-image-elements,agroup/tripleo-puppet-elements,rdo-management/tripleo-image-elements,qwefi/tripleo-image-elements,rdo-management/tripleo-image-elements,openstack/tripleo-image-elements,qwefi/tripleo-image-elements,agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,radez/tripleo-image-elements,agroup/tripleo-puppet-elements