Use eth0 as dhcp_interface in cobbler container

Docker containers have only "eth0" and "lo" interfaces by default.
So if we deploy fuel on any non "eth0" interface, then
/etc/dnsmasq.conf file will have worng "interface" parameter which
breaks DHCP discovery. We need to hardcode dhcp_interface=eth0.

Change-Id: Iae32f1139fffca1cb77484978d07631ceed4fcee
Closes-bug: #1319412