This document previously existed to explain how to use either
application/json or application/xml with Nova. It's mostly confusing
in it's current state, and we're correctly defaulting to
application/json on request and response now.
Just remove most of what's confusing, users don't need to send
headers, we only do one thing now.
Change-Id: I597aeab73f1075f400f053016b4ea7bda01893a5