guacamole-rdp-vnc-gateway-existing-vnet - referenced docker image is very out of date and is no longer the main branch

[guacamole-rdp-vnc-gateway-existing-vnet](https://github.com/Azure/azure-quickstart-templates/tree/master/guacamole-rdp-vnc-gateway-existing-vnet)

### Issue Details
The referenced docker image in the cloud-config.yaml is out of date by several versions and has not been updated in 2 years.

Reference to both guacamole and guacd need updated.

Old Docker Ref - ExecStartPre=-/usr/bin/docker pull glyptodon/guacamole:latest
ExecStartPre=-/usr/bin/docker pull glyptodon/guacd:latest
New Docker Ref - ExecStartPre=-/usr/bin/docker pull guacamole/guacamole:latest
ExecStartPre=-/usr/bin/docker pull guacamole/guacd:latest

Old Docker Hub Link: https://hub.docker.com/r/glyptodon/
New Docker Hub Link: https://hub.docker.com/r/guacamole/

Open guacamole-rdp-vnc-gateway-existing-vnet/cloud-config.yaml and inspect the Docker pull references for guacamole and guacd. Update both references from glyptodon to the guacamole Docker Hub namespace, then verify that no old references remain in the template.

Written by the indexing model from the issue text.