Flag private-key attribute as sensitive

[agrevtsev]

Trying to fix #677
as it seems terraform plan is leaking full-body private_key

Terraform detected the following changes made outside of Terraform since the
last "terraform apply" which may have affected this plan:

  # openstack_compute_keypair_v2.keypair_1 has been deleted
  - resource "openstack_compute_keypair_v2" "keypair_1" {
      - id          = "prod-prod-1-keypair_1" -> null
        name        = "prod-prod-1-keypair_1"
      - private_key = <<-EOT
            -----BEGIN RSA PRIVATE KEY-----
            <skip-skip-skip-skip>
            -----END RSA PRIVATE KEY-----
        EOT -> null
      - public_key  = "ssh-rsa <skip-skip-skip-skip> Generated-by-Nova" -> null
        # (1 unchanged attribute hidden)
    }