fail with useful error on unmatched admin interface (bsc#990745)

Catch the case where we failed to find an admin interface for a node,
and bail with a more helpful error message.

This was experienced in a scenario where the Crowbar nodes were a mix of
physical machines and VMs with slow virtio interfaces, but the
network.json was incorrectly configured and the admin network conduit
failed to match the slow virtio interfaces.

https://bugzilla.suse.com/show_bug.cgi?id=990745

chef/cookbooks/provisioner/recipes/update_nodes.rb

@@ -20,6 +20,10 @@ def find_node_boot_mac_addresses(node, admin_data_net)
   result = []
   admin_interfaces = admin_data_net.interface_list
   admin_interfaces.each do |interface|
+    if interface.nil?
+      raise "Failed to find admin node interface for node #{node.fqdn}! " \
+            "Check your network barclamp proposal."
+    end
     node["network"]["interfaces"][interface]["addresses"].each do |addr, addr_data|
       next if addr_data["family"] != "lladdr"
       result << addr unless result.include? addr