Improve install guide keystone verify section

Improve the keystone verify section of the installation guide
as follows:

1) Clarify steps to test operation for both admin and normal
   users.
2) Add step to show that normal users cannot run admin-only
   commands.
3) Provide more links to useful topics.
4) Adjust titles.
5) Create temporary workaround for Debian issues.

This patch implements changes approved for other patches associated
with this blueprint. I will provide instructions to create openrc
files in a separate patch.

Implements: blueprint installation-guide-improvements
Change-Id: I42ebc321ef39911b090f450ff49bb6e1a1e9bf4e
Merge "Improve install guide keystone verify section"