Fix delete default namespace

[lgfa29]

Currently if you reference the default namespace in your configuration Terraform will fail to destroy it because the API will return a 500 error:

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  - destroy

Terraform will perform the following actions:

  # nomad_namespace.default will be destroyed
  - resource "nomad_namespace" "default" {
      - description = "Default shared namespace" -> null
      - id          = "default" -> null
      - name        = "default" -> null
    }

Plan: 0 to add, 0 to change, 1 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

nomad_namespace.default: Destroying... [id=default]

Error: error deleting namespace "default": Unexpected response code: 500 (can not delete default namespace)

This error can prevent other resources from being destroyed if they depend on this namespace.

This PR prevents this error from happening by not trying to delete the default namespace. Instead it resets the quota and description attributes to their initial values.

Fixes #72 .

[cgbaker]

manually tested, works great.
minor nitpicks, but lgtm!

[cgbaker]

👍

[cgbaker]

👍