Replace null resource in terraform template with locals to fix warning

[Matt343]

Fixes #1874

This replaces the null_data_source used in the terraform template with locals.

It is a breaking change for anyone who was using the old values in their templates. I can change this to just the addition of the locals if we prefer to avoid the API change for now, that would at least give people an alternative although it would not resolve the warning yet.

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