Add deprecation notice for support of Terraform v0.11

[bmelbourne]

As of Nov 9th 2019, HashiCorp is starting a phased approach to deprecate support of Terraform v0.11 in Terraform providers.

https://www.hashicorp.com/blog/deprecating-terraform-0-11-support-in-terraform-providers

This deprecation notice should be merged in kops v1.19 prior to the merge of PR #9940 in kops v1.20.

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[hakman]

/ok-to-test
/lgtm
/assign @rifelpet

[bmelbourne]

/assign @rdrgmnzs

[rifelpet]

/approve

Thanks for doing this! I wonder if we should also log a warning in kops 1.19 when someone uses the deprecated behavior via -Terraform-0.12.

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This pull-request has been approved by: bmelbourne, rifelpet

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[hakman]

@bmelbourne I think your branch is missing #9988. Could you rebase so that docs tests pass?
/lgtm cancel

[hakman]

Thanks @bmelbourne .
/lgtm