provider/google: Add google_compute_zones data source

d/google_compute_zones.html.markdown

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+---
+layout: "google"
+page_title: "Google: google_compute_zones"
+sidebar_current: "docs-google-datasource-compute-zones"
+description: |-
+  Provides a list of available Google Compute zones
+---
+
+# google\_compute\_zones
+
+Provides access to available Google Compute zones in a region for a given project.
+See more about [regions and zones](https://cloud.google.com/compute/docs/regions-zones/regions-zones) in the upstream docs.
+
+```
+data "google_compute_zones" "available" {}
+
+resource "google_compute_instance_group_manager" "foo" {
+  count = "${length(data.google_compute_zones.available.names)}"
+
+  name               = "terraform-test-${count.index}"
+  instance_template  = "${google_compute_instance_template.foobar.self_link}"
+  base_instance_name = "foobar-${count.index}"
+  zone               = "${data.google_compute_zones.available.names[count.index]}"
+  target_size        = 1
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `region` (Optional) - Region from which to list available zones. Defaults to region declared in the provider.
+* `status` (Optional) - Allows to filter list of zones based on their current status. Status can be either `UP` or `DOWN`.
+  Defaults to no filtering (all available zones - both `UP` and `DOWN`).
+
+## Attributes Reference
+
+The following attribute is exported:
+
+* `names` - A list of zones available in the given region