chef-server.rb changes

Two changes:

(1) add -p to mkdir when setting up /etc/opscode.  Not because there are any parent directories (if you're missing /etc, well, you've got bigger issues), but because -p doesn't error out if the directory already exists, it just silently does nothing (which is the behavior we want).

(2) changed reconfigure to overwrite /etc/opscode/chef-server.rb with /etc/opscode/private-chef.rb if chef-server.rb doesn't exist (old behavior) and also if chef-server.rb is empty (new behavior).