Add $http_additional_array parameter to nginx module.

The $http_additional_array parameter takes an array of key/value pairs for use in passing additional configuration to the http{} block of nginx.conf. The avoids the need to add a new parameter for every configuration directive required in our setup. Previously I was handling this by dropping a file into /etc/nginx/conf.d/ via hubspot::roles::nginx::role_elb. However an issue arose with access_log where a log_format was being specified before it was defined which was causing nginx to fail to start. Instead of adding a log_format parameter to the class I just added this parameter and killed of the file from hubspot::roles::nginx::role_elb.