Add format_log option for nginx proxies

[jonjhallettuob]

We are now pushing our nginx logs to Logstash and Kibana. We'd like the logs to include the virtual host and time taken. Currently we can't do that for eXistdb's nginx logs, because there is no way of setting the format_log option to nginx::resource::server.

Please add a format_log option to existdb::reverseproxy::server so a log format can be passed in through Hiera.