Add new key to control reattach disk to vm

[vishesh92]

Fixes #72

[rohityadavcloud]

LGTM didn't test it

[kiranchavala]

LGTM, tested it manually and, the resize operation happened without any detach of the disk

  # cloudstack_disk.test-disk will be updated in-place
  ~ resource "cloudstack_disk" "test-disk" {
        id                 = "9d87be3a-1199-4cee-b491-3ef4edbec428"
        name               = "test-disk"
      ~ size               = 15 -> 20
        tags               = {}
        # (7 unchanged attributes hidden)
    }

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