Crude support for chef-server 12

Hackish approach to supporting chef server 12 by adding an attribute for the /etc/ path in which to write the chef-server.rb file.

To use to install Chef Server 12, set:

default['chef-server']['package_file'] = '...' # Some chef-server 12 RPM or DEB URL
default['chef-server']['config_path'] = '/etc/opscode'

I don't really expect this to get merged in, as there is surely a better / more official way.  I didn't want to implement logic around the version, since I don't control what :latest means, etc, and someone is surely working on this somewhere.