Guacamole desktop support for Ubuntu

Fixes #501 (closed).

Todo

• Update guidance text on create server page, say that Ubuntu is also supported
• Figure out why VNC server needs a manual restart after unshelving

How to Test

First, set in your config.js:

  "instanceConfigMgtRepoUrl":"https://gitlab.com/cmart/exosphere.git",
  "instanceConfigMgtRepoCheckout":"issue-501"

I tested with image "focal-server-cloudimg-amd64-with-desktop-minimal" owned by project TG-CCR190024 on IU cloud.

• Launch from that instance, ensure Guacamole desktop works. Shelve it, unshelve, ensure it still works after instance comes back online.
• Ensure Guacamole desktop on CentOS 8 still works (it does in my testing)

Screenshots

QA Checklist

• Any changes to 'widgets' in src/Style/Widgets/? If so:
  • Update src/Style/StyleGuide.elm showing example usage of that widget
• Were any app flags created, modified, or removed? If so:
  • Update config.js and all files in environment-configs/, including docker-config.js
  • Update "Runtime configuration options" documentation in README.md

Post-Merge Checklist

• Create follow-up issues for anything left over for another merge request
  • If any technical debt is introduced, assign these follow-up issues to the MR submitter