Pull request - ganetiviz application (adds graphical visualization for a ganeti cluster)

* Added a new application **ganetiviz** (loosely coupled)  in ganeti_webmgr/ganeti_web/
* Re-factored ganetiviz specific code into a separate application including, views, urls, static files.
* Added a single template file in ganetiviz/templates/
* pramttl:feature/13569 rebased over osuosl:develop successfully to to pull in latest changes before creating this pull request
* Everything in GWM works well, tested.
* Unable to test-run "ganetiviz" along with GWM since this requries merge in of: 
(ganeti_webmgr/pull/3)[https://github.com/osuosl/ganeti_webmgr/pull/3]