Allow empty hostgroups by default

This allows the first Chef-managed icinga server to boot properly.

Not sure why, but a new node's run_list doesn't appear to be populated on the Chef server until the node has successfully run Chef with that run_list. Unfortunately, this is a chicken-and-egg problem unless empty hostgroups are allowed -- the node['icinga']['server_role'] list will be empty (unless another server with that role is already running), causing Icinga startup to fail, causing Chef to fail, causing the run_list to remain unpopulated.

Option docs:
http://docs.icinga.org/latest/en/configmain.html#configmain-allow_empty_hostgroup_assignment