Ignore missing variables during destroy phase

[radeksimko]

This is fixing #2892

I think Mitchell aimed to fix it in https://github.com/hashicorp/terraform/pull/2775/files#diff-6baa6e1e203f1aabf89e2169ec8c89b1R373 but it only covers plan -destroy, not the actual destroy.

I'm not quite sure why there was no walkDestroy operation until now - if there's an obvious reason that I don't see, please tell me.

[phinze]

Thanks for digging into this, @radeksimko!

This looks solid. I talked with @mitchellh about our prior lack of a walkDestroy, and we both agreed that there was no specific reason, and if all the tests pass this seemed okay to do.

Worth noting that previously Destroy: true on the GraphBuilder was used to determine this - I'm guessing we can probably drop that consolidate around operation now that this is landing.

But for now, this LGTM! 🚢

[ghost]

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