FIX: config output must not contain `gs://` prefix
FIX: config output must not contain `gs://` prefix

Currently:
```
terraform {
  backend "gcs" {
    bucket = "gs://<BUCKET_NAME>"
    prefix = "."
  }
}
```

Desired:
```
terraform {
  backend "gcs" {
    bucket = "<BUCKET_NAME>"
    prefix = "."
  }
}
```
GoogleCloudPlatform/terraformer