DHCP assignment with multiple nics

When multiple ethernet adapters are presend, a host file is created in /eth/dhcp3/hosts.d/ for each of the interfaces. Each of those files contained the same IP address

* Added condition in update_nodes.rb to set IP address only for the boot interface
* added condition in dhcp template to deny boot if no IP is set (see update_nodes.rb)
Merge pull request #3 from gabriel-samfira/release/pebbles/master

DHCP assignment with multiple nics