[ISSUE] GS mount provider cannot handle mounting cluster deletion

[marekbrysa]

Configuration

 resource "databricks_mount" "foobucket" {
      name       = "foobucket"
      gs {
          bucket_name     = "foobucket"
          service_account = "fooserviceaccount@fooproject.iam.gserviceaccount.com"
        }
    }

Expected Behavior

After this configuration is applied and the mounting cluster manually (accidentally or by expiration) deleted, the template can still be applied without errors.

Actual Behavior

cluster_id of the mounting cluster is stored in tf state such as "cluster_id": "0524-131054-9zs18rj2", which makes tf apply fail with
Error: cannot get mounting cluster: Cluster 0524-131054-9zs18rj2 does not exist because the code goes into this branch of code.

The "fix" is to manually edit tf state and set `"cluster_id": "". Then the cluster is created by tf again.

Steps to Reproduce

Please list the steps required to reproduce the issue, for example:

1. terraform apply
2. Delete the mounting cluster manually in Databricks workspace.
3. terraform apply

Terraform and provider versions

Terraform Version: 0.12
Provider "gcp_databricks" with version "~> 0.5.4"