Improved installation guide glance section

I improved the glance installation and configuration section of
the installation guide as follows:

1) Restructured steps to improve flow/clarity and consistency
   with similar sections.
2) Sourced admin-openrc.sh.
3) Adjusted titles.

Change-Id: Ic69835e7f3af123c8c537b75661a8ac4651c15e7
Implements: blueprint installation-guide-improvements
Merge "Improved installation guide glance section"