Subnetwork: Infers region from zone before using the provider-level region

[rosbo]

Fixes #926

Affects resources with a zone field in its schema and using the getRegion method:

• google_compute_instance
• google_compute_disk
• google_compute_instance_template

This change affects how region is inferred.

The main goal is to change how the region is inferred when a subnetwork is set using the name only (not a self_link).

We used to fallback to the provider-level region first. I changed that to infer the region from the zone in the schema first.

For example, given this config:

resource "google_compute_instance" "default" {
  name = "test"
  zone = "asia-northeast1-b"
  machine_type = "g1-small"

  boot_disk {
    initialize_params {
      image = "${var.instance_image}"
      type = "pd-ssd"
    }
  }

  network_interface {
    subnetwork = "mirror"
    # Before, wrong region 
   # After, region is
  }
}

provider "google" {
  project = "${var.provider_project}"
  region = "us-central1"
} 

Before this change, the `subnetwork` used would be `projects/a-project/regions/us-central1/subnetworks/mirror`. 

After this change, the `subnetwork` used would be `projects/a-project/regions/asia-northeast1/subnetworks/mirror` which is what we want.

This should not break anybody because the region should match the zone for existing resource. It does help when people create new config using only the name for regional resources.

[danawillow]

nit: doesn't the "ok" mean that it isn't empty?

[rosbo]

Yes:
// GetOk returns the data for the given key and whether or not the key
// has been set to a non-zero value at some point.

The check on regionSchemaField is for cases where the field name is set to "" to basically skip checking the schema.

[danawillow]

oh oh oh I was reading it wrong, I see now

[ghost]

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