prevent TF 0.12 warning by updating syntax

[displague]

This PR removes the TF 0.11 style string interpolation within the Anthos template files.

This was missed by https://github.com/packet-labs/google-anthos/pull/76/ because of the tf.tpl extension in use (for the nested terraform apply)

This change was formatted with terraform fmt, to see the changes with less white-space noise, review with ?w=1: https://github.com/packet-labs/google-anthos/pull/92/files?w=1

null_resource.anthos_deploy_cluster[0] (remote-exec): Warning: Interpolation-only expressions are deprecated

null_resource.anthos_deploy_cluster[0] (remote-exec):   on deploy_cluster.tf line 8, in resource "null_resource" "anthos_deploy_cluster":
null_resource.anthos_deploy_cluster[0] (remote-exec):    8:         private_key = "${file("/root/anthos/ssh_key")}"
null_resource.anthos_deploy_cluster[0] (remote-exec):
null_resource.anthos_deploy_cluster[0] (remote-exec): Terraform 0.11 and earlier required all non-constant expressions to be
null_resource.anthos_deploy_cluster[0] (remote-exec): provided via interpolation syntax, but this pattern is now deprecated. To
null_resource.anthos_deploy_cluster[0] (remote-exec): silence this warning, remove the "${ sequence from the start and the }"
null_resource.anthos_deploy_cluster[0] (remote-exec): sequence from the end of this expression, leaving just the inner expression.

null_resource.anthos_deploy_cluster[0] (remote-exec): Template interpolation syntax is still used to construct strings from
null_resource.anthos_deploy_cluster[0] (remote-exec): expressions when the template includes multiple interpolation sequences or a
null_resource.anthos_deploy_cluster[0] (remote-exec): mixture of literal strings and interpolations. This deprecation applies only
null_resource.anthos_deploy_cluster[0] (remote-exec): to templates that consist entirely of a single interpolation sequence.

null_resource.anthos_deploy_cluster[0] (remote-exec): (and one more similar warning elsewhere)

[geojaz]

/lgtm I've tested this change locally.

[paemason]

I've manually tested this as well. LGTM wil merge.