Update nginx config to match latest docs

[tleyden]

The nginx config should match the latest docs as closely as possible:

https://developer.couchbase.com/documentation/mobile/1.5/guides/sync-gateway/nginx/index.html#basic-nginx-configuration-for-sync-gateway

This might be the reason the testfest nginx setup didn't work recently.

Also, it's possible that the updated config has introduced new issues, and those should be tested.

NOTE: sorry if this is a duplicate ticket, I can't find the first one I filed.