cli: allow disabling "next steps" message in terraform plan

[apparentlymart]

In #15884 we adjusted the plan output to give an explicit command to run to apply a plan, whereas before this command was just alluded to in the prose.

Since releasing that, we've got good feedback that it's confusing to include such direct instructions when Terraform is running in a workflow automation tool, because such tools usually abstract away exactly what commands are run and often require users to take different actions to proceed through the workflow.

To accommodate such environments while retaining helpful messages for normal CLI usage, here we introduce a new environment variable TF_IN_AUTOMATION which, when set to a non-empty value, is a hint to Terraform that it isn't being run in an interactive command shell and it should thus tone down this sort of "next steps" messaging.

The documentation for this setting is included as part of the "...in automation" guide since it's not generally useful in other cases. We also intentionally disclaim comprehensive support for this since we want to avoid creating an extreme number of "if running in automation..." codepaths that would grow the testing matrix and hurt maintainability.

The focus is specifically on the output of the three commands we give in the automation guide, which at present means the following two situations:

• terraform init does not include the final paragraphs that suggest running terraform plan and tell you in what situations you might need to re-run terraform init.
• terraform plan does not include the final paragraphs that either warn about not specifying -out=... or instruct to run terraform apply with the generated plan file.

[jbardin]

I think in the long run it might be cleaner to have another CLI implementation that handles the output appropriately, but I think that would require extending the CLI interface to change the "Output" method, or add another one to differentiate what is being shown.

I don't see any problem with this for now though.

[apparentlymart]

Indeed I think with the limited focus this has for now (only on the commands we're documenting for use in automation) this less-invasive change is best, but in the long run we may want to diverge these changes more. At that point though we'd probably transition to machine-readable output for automation, once we're confident the system design is stable enough that we can maintain such an interface.

[ghost]

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