Adapt to gardener/terraformer@v1.0.0

[rfranzke]

What this PR does / why we need it:
Adapt terraformer package to v1.0 of gardener/terraformer.

Special notes for your reviewer:

• ❌Requires merge of Live configmap lookup to cater with stale kubelet cache gardener/terraformer#37
• ❌Release of gardener/terraformer

Release note:

When using this version of the Gardener Extensions library you have to make sure that you use at least `v1.0` of the `gardener/terraformer` image.

[ialidzhikov]

I guess terraformer rbac needs to be updated

Error from server (Forbidden): secrets "bar.infra.tf-vars" is forbidden: User "system:serviceaccount:shoot--foo--bar:terraformer" cannot get resource "secrets" in API group "" in the namespace "shoot--foo--bar"

Btw what is the idea of this secret as its content is always # New line is needed! Do not remove this comment.?

[rfranzke]

Yes, will do, nice catch, thanks.

The secret contains the Terraform variables that themselves might include credentials. It's up to the caller to fill them. In our Gardener Extensions we decided to not use them, that's why the file is always empty (just has this comment). Generally, we don't know how others are calling or deploying our Terraformer. Though, as TF variables are a first-class citizen in the TF project I guess it makes sense to support it.

[rfranzke]

/close as @ialidzhikov will incorporate the changes of this PR into his, #624