For users who in previous versions have relied on our lack of checking for
whether variables are declared, they may previously have seen an
overwhelming number of warnings when running Terraform v0.12.
Here we cap that number at three specific warnings and then one general
warning, so we can still give a specific source location for the first
couple (for users who have genuinely made a typo) but summarize away a
large number for those who are seeing this because they've not yet
migrated to using environment variables.
Martin Atkins