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
(cherry picked from commit 4f8a32470d476b45a4de3f9ec66d39b4021914eb)
Merge "Clarify heat roles in the Installation Guide" into stable/juno
Clarify heat roles

Building on an earlier patch, I further clarified the purpose
of the heat_stack_owner and heat_stack_user roles.

Change-Id: I67804d2e7bfbd53e8f453adc251a102c6f0e39ff
Closes-Bug: #1401668
backport: juno
(cherry picked from commit 01a4e4060b300c1fc0b1b1634f106115e79aef2b)