Improve install guide keystone users section

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

1) Clarify use of ADMIN_TOKEN with example.
2) Replace procedures with steps/substeps to reduce bulk.
3) Add example output.
4) Adjust titles.
5) Provide more links to useful topics.

Implements: blueprint installation-guide-improvements

Change-Id: I4b6f30dfd60ba95fc5d4b218c07d5f9cdeaaabc6
Merge "Improve install guide keystone users section"