This is the config we use to talk to Cloudant without changing any of our existing services.
It requires two things - the hostname and the Basic Auth header and then it is good to go. To generate the basic auth:
echo -n "username:password" | openssl enc -base64
Then pop that in set header Authorization following the word Basic. That's it.
Tested with nginx 1.1.19.
We figured this might be useful to others who migrate from CouchDB to Cloudant