documentation for audit middleware

this moves (and edits) audit middleware documentation from pycadf
library to keystonemiddleware

pycadf doc: https://github.com/openstack/pycadf/blob/master/doc/source/middleware.rst

Implements: blueprint audit-middleware
Change-Id: I068f312d8927010fd209eab5c22910c4d1d343a1
Merge "documentation for audit middleware"