Resolve accidental destruction of startup-scripts

[tpdownes]

When the startup-script module is used with VM instance templates that have create_before_destroy enabled (a common setting), a change to the script contents will result in the script being created, then destroyed. This commit resolves that by ensuring that all startup-script objects are created with unique, repeatable identifiers. This allows the new object to be created while destroying the old object with a different identifier.

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[tpdownes]

This PR will be followed up by another PR to update all internal use of startup-script to the appropriate ref.

When testing, please give extra consideration to runners specified with "source" rather than "content" as those are less covered in tests.

[tpdownes]

An additional test that I performed but you should also consider: trigger a change to the template that does not involve the script and observe that nothing changes with the script. e.g. machine_type.