Improved install guide nova compute section

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

1) Restructured steps to improve flow/clarity and consistency with
   similar sections.
2) Adjusted titles.

Most of these improvements incorporate changes approved for other
patches associated with this blueprint.

Co-Authored-By: Stephen Gordon <sgordon@redhat.com>
Change-Id: I311a104b928549badec2a9ea4637f72bde084324
Implements: blueprint installation-guide-improvements
Merge "Improved install guide nova compute section"