Clarify heat roles in the Installation Guide

1. Added a brief description of the heat_stack_owner and heat_stack_user roles
2. Added a warning not to assign heat_stack_owner and heat_stack_user roles to the same user.

Change-Id: Ic180902bfe2d2e66eb8739f7ef41f6dd96b11d6b
backport: Juno
Closes-Bug: #1401668
Merge "Clarify heat roles in the Installation Guide"