Improvements to the dashboard installation section

Improved dashboard content in the installation guide as
follows:

1) Restructured steps for consistency with similar sections.
2) Updated list of packages to install.
3) Adjusted configuration on all distributions to use memcached
   for session storage to simplify installation.
4) Clarified instructions for access permissions.
5) Split verification step into separate section for consistency
   with other chapters.
6) Changed removal of Ubuntu theme package from 'required' to
   'optional' based on user experience.
7) Added workaround for RDO packaging issues.

This patch includes changes for Juno.

Change-Id: Iecb1cff57853b097ba2ee0a5016438190ff8d920
Implements: blueprint installation-guide-improvements
Co-Authored-By: Matt Kassawara <mkassawara@gmail.com>
Merge "Improvements to the dashboard installation section"