DHCP assignment with multiple nics

This is https://github.com/crowbar/barclamp-provisioner/pull/211 for roxy.

When multiple ethernet adapters are present, a host file is created in /etc/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)