Edit Identity Service chapter of Cloud Admin Guide

Moves authentication middleware info from the logging subsection to the authentication middleware section
Adds info on domains to the concepts section
Updates grammar and style in the affected sections

Change-Id: I96858e4b5ebadbcce8872b08992a00a9d638d1a4
author: nermina miller
backport: havana
Closes-Bug: #1239359
Merge "Edit Identity Service chapter of Cloud Admin Guide"