Unable to specify category key names dynamically in resource:nutanix_virtual_machine

[rdobrynski]

Describe the bug
When defining a nutanix_virtual_machine resource, using an expression to assign a category key fails. The expression does not expand to it's value.

Expected behavior
expression for category key should expand to it's value, instead of pass the literal expression to prism central as the category key name.

Logs
Error: Error applying plan:

1 error occurred:
* nutanix_virtual_machine.vm1: 1 error occurred:
* nutanix_virtual_machine.vm1: error: {
"api_version": "3.1",
"code": 404,
"kind": "vm",
"message_list": [
{
"message": "Category ${var.test_category_key}: does not exist.",
"reason": "ENTITY_NOT_FOUND"
}
],
"state": "ERROR"
}
Versions (please complete the following information):

• OS that is executing Terraform
  MacOS Mojave 10.14.5

• Terraform
  0.11.14

• Nutanix Cluster (Prism Element / AOS)
  5.10.4 LTS

• Nutanix Prism Central
  5.10.4

• Terraform provider version (using posted RC1? Compiling yourself? If so, from Develop or Master, and what commit id?
  provider.nutanix: version = "~> 1.0"

Additional context
Add any other context about the problem here.