Guacamole Linux VM cloud-init config file contains hardcoded Ubuntu version

[SvenAelterman]

Describe the bug

In apt_sources_config.yml, the entry for the Microsoft package manager contains a hardcoded Ubuntu version 18.04 but a $RELEASE placeholder. When combined and written to sources.list, this results in impossible combinations, such as .../18.04/prod focal.. (where focal is the name of Ubuntu 20)

This means that a single bundle cannot be used for multiple Ubuntu releases, and certainly not for multiple Linux distributions.

Steps to reproduce

1. Update template_schema.json and porter.yaml for a recent Linux version.
2. Publish the bundle.
3. Attempt to deploy a Linux VM with the newer image.

Result: apt-get fails during cloud-init, leaving the VM inaccessible from Guacamole.

Azure TRE release version (e.g. v0.14.0 or main): v0.15.2

[tim-p-allen]

Looking at fixing this with #3893