[Terraform]: google_bigtable_instance add back validation to num_nodes.

[rileykarson]

Fixes hashicorp/terraform-provider-google#2400

Wow! This one is weird.

Given a config like this (we're implicitly making a PRODUCTION instance with a TF default):

resource "google_bigtable_instance" "instance" {
  name = "foo"

  cluster {
    cluster_id   = "foo"
    zone         = "${local.zone}"
    num_nodes    = {{NUMBER}}
    storage_type = "SSD"
  }
}

• NUMBER = 0: Fails saying that a PRODUCTION instance needs some number of nodes specified
• NUMBER = 2: Fails saying that you need at least 3 nodes
• NUMBER = 3: Works as expected

All of these make sense, but then...

• NUMBER = 1: Creates a DEVELOPMENT instance (in a request where we explicitly requested a PRODUCTION instance!) which has no num_nodes because DEVELOPMENT instances don't have num_nodes.

This validation means num_nodes can't be specified as 0, which is technically a breaking change but leaving it unspecified works fine.

---

[all]

[terraform]

google_bigtable_instance add back validation to num_nodes.

[terraform-beta]

[ansible]

[inspec]

[modular-magician]

I am a robot that works on MagicModules PRs!

I built this PR into one or more PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#82
depends: hashicorp/terraform-provider-google#2401

[modular-magician]

I am (still) a robot that works on MagicModules PRs!

I just wanted to let you know that your changes (as of commit 5ef7fb8) have been included in your existing downstream PRs.