Improved Installation Guide keystone install section

1. Added text to indicate installing the Identity service is on the controller node.
2. Reordered steps in the procedure so the database is created before installing packages.

Implements: blueprint installation-guide-improvements
Co-Authored-By: Matt Kassawara <mkassawara@gmail.com>

Change-Id: I3dc3dddf3073407c2d7e0b56f2b36255f11d7d56
Merge "Improved Installation Guide keystone install section"