"Ganetiviz" Cluster Visualization app added - Ready for pull into "develop"

Hi,

I have added the "ganetiviz" cluster mapping / visualization application to GWM as per [feature #13569](https://code.osuosl.org/projects/ganeti-webmgr/issues/13569)

In GWM, the visual cluster map is generated by visiting the url: 
    
    /map/<cluster_hostname>

**Example**
    localhost:8000/map/ganeti.example.org

Explanations on using the front-end are available in the README of the front-end component of this "ganetiviz" feature, here - [ganetiviz-cytoscape](https://github.com/pramttl/ganetiviz-cytoscape)

The commit summary should give a succinct view of the changes. Hope commit messages are good. :)