Add auto-delete boot disk as an option on vm-instance

[nick-stroud]

If even only for testing, I often want to turn auto-delete off. This exposes the option without changing the default behavior.

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[tpdownes]

Although I understand the value in broad strokes, it's not clear to me what workflow you are trying to enable. In particular, the way I read the code, updating var.startup_script should not cause the VM to be deleted/re-created.

[nick-stroud]

it's not clear to me what workflow you are trying to enable.

In the current state, If I have waited a long time for startup scripts to install something and then I want to tweak a machine parameter that forces replacement, I will lose the disk and have to wait for startup to run again.

[tpdownes]

I have some concerns about effectively managing the destruction of the disk twice (explicitly and implicitly) that this change does not address. I also suspect you could address some of your concerns by shutting the machine down, running Terraform, and then turning it back on.

That said, it's a knob that arguably merits existence.

In medium term I think we should probably manage the creation and destruction of the boot disk entirely implicitly (with auto deletion either on or off) and allow the user to supply externally-created secondary disks.