Automatically update connection database to reflect available VMs

[jemrobinson]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

When we add/remove workspace VMs we have to update the Guacamole connection database (code is in provisioning > sre_provisioning_manager.py). We should make this automatic, instead of a separate step done at the end of dsh deploy sre.

🚂 How could this be done?

This could use e.g. Azure Functions or similar.