Don't set metadata_startup_script in some cases.

[nat-henderson]

If the state contains metadata.startup-script, the user likely wants to keep it in there instead of metadata_startup_script (which is ForceNew, where metadata.startup-script is not). This isn't the standard case - metadata.startup-script creates heterogeneous fleets unless managed very carefully, which is why metadata_startup_script exists - but it's supported by the API and it's a valid state, so we want it to continue working.

After this PR:

• If you set metadata_startup_script and don't set metadata.startup-script, you will see no change.
• If you set metadata.startup-script and don't set metadata_startup_script, you will stop seeing permanent diffs, because Read will stop setting metadata_startup_script.
• If you set both, your behavior will be strange and you'll probably see permanent diffs - don't set both.
• If you do an import, you'll get metadata_startup_script.

If you're affected by something like #1079, this will make sure that you don't have problems going for anything created after this PR goes in, but for your existing instances you'll need to make sure that the metadata.startup-script has a non-empty value, probably using the terraform state functions.

[rosbo]

If you set both, your behavior will be strange and you'll probably see permanent diffs - don't set both.

Should we add a ConflictsWith to ensure people don't set both?

[nat-henderson]

Can't, unfortunately - metadata is a map. Probably need to put it in the validator - I'll take a look at that. If it's easy I'll add it to this PR.

[nat-henderson]

Shoot - I've got a problem with updates from metadata.startup-script -> metadata_startup_script. I'll try and fix it...

[nat-henderson]

Got it. Running the tests.

[nat-henderson]

All the tests passed, merging.

[ghost]

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