Improved install guide nova controller section

I improved the nova controller section of the installation guide
as follows:

1) Restructured steps to improve flow/clarity and consistency with
   similar sections.
2) Explicitly sourced admin-openrc.sh.
3) Explicitly configured 'glance_host' key.
4) Adjusted titles.
5) Split single verify step into separate section after installation
   on compute node and added another command to run.

Most of these improvements incorporate changes approved for other
patches associated with this blueprint. A future patch will cover
improvements to the nova compute node section.

Change-Id: I12bc6c4f2a05a55ecbec3b17e3477a002cde4e1a
Implements: blueprint installation-guide-improvements
Merge "Improved install guide nova controller section"