Virtual Machine improvements

[ninjarobot]

The changes in this PR are as follows:

• Virtual Machines: Use an Azure-managed storage account for boot diagnostics.
• Virtual Machines: Create a VM without any data disks at all (useful when mounting cloud storage).
• Virtual Machines: Adds support for Ubuntu 20.04 OS image.

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

vm {
    name "demovm"
    username "farmeruser"
    vm_size Standard_B1ms
    operating_system UbuntuServer_2004LTS
    diagnostics_support_managed
    no_data_disk
}

[isaacabraham]

LGTM :-)

[ninjarobot]

Thanks for the enhancement, this is now released in 1.7.5 and available on nuget.