Install Guide: fix heat endpoints

Use the same syntax as the other services for Heat endpoints creation in
keystone.

Change-Id: Id6e04919ee154b2c1c692369eced31d69a016570
backport: havana
Update headings, edits for consistency/clarity/ Install Guide

Partial-Bug: #1250515

backport: havana

Change-Id: I11117af7e13966f70e0050f96a7b2a1c4bcb59ca
author: diane fleming
Merge "Install Guide: fix heat endpoints"