make create idempotent

[JonahBraun]

make create Can now be run twice in a row without error. Also, a create can be run after a teardown. To test:

make create
# Optionally modify TF files.
make create
# Should finish successfully, applying TF changes if any.

make teardown
make create

I recommend we also make make teardown idempotent. Currently it fails if the terraform project has not been initialized. However, it should detect this and continue to clean any other remains of of past use, deleting terraform.tfvars, etc. and exiting cleanly.

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[elungwu]

echo "${TFVARS_FILE} already exists, skipping generation."
^ Instead of skipping generation, should we remove and generate a new one just in case the existing one has different data.

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[google-oss-robot]

@elung-anthos: changing LGTM is restricted to collaborators

In response to this:

echo "${TFVARS_FILE} already exists, skipping generation."
^ Instead of skipping generation, should we remove and generate a new one just in case the existing one has different data.

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[helmsman-bot]

Can one of the admins verify this patch?