Change default case nic in post_install_network_config_fuel

Removes long wait times in centos on first boot when devices are left
 in DHCP mode and have to wait for timeout.

Resolves-bug #1260879

Change-Id: Iaf4f2fa55ba96cc9298cad02a67e2309c74e9cb6
Merge "Change default case nic in post_install_network_config_fuel"