when removing node, remove configs for all his dhcp hosts

Currently when node's NIC is removed and crowbar does not know about it, it won't delete its dhcp config when the node is being deleted (the code only searches for current MAC addresses of the node).

As a result, forgotten + rebooted node is not PXE-booted

SUSE reference: https://bugzilla.novell.com/show_bug.cgi?id=842442