Haproxy Fixes

* Use `attributes/customize.rb` (https://github.com/aws/opsworks-cookbooks/blob/master-chef-11.10/haproxy/attributes/default.rb#L1-L14)
* Generate our config file only if our error files have been created (https://github.com/easybib/issues/issues/1943#issuecomment-52456469)