provider/google: Add support for backend buckets

Adds a new resource google_compute_backend_bucket according to
https://cloud.google.com/compute/docs/reference/latest/backendBuckets

Fixes hashicorp/terraform#12505

r/compute_backend_bucket.html.markdown

@@ -0,0 +1,52 @@
+---
+layout: "google"
+page_title: "Google: google_compute_backend_bucket"
+sidebar_current: "docs-google-compute-backend-bucket"
+description: |-
+  Creates a Backend Bucket resource for Google Compute Engine.
+---
+
+# google\_compute\_backend\_bucket
+
+A Backend Bucket defines a Google Cloud Storage bucket that will serve traffic through Google Cloud
+Load Balancer.
+
+## Example Usage
+
+```hcl
+resource "google_compute_backend_bucket" "foobar" {
+  name        = "image-backend-bucket"
+  description = "Contains beautiful images"
+  bucket_name = "${google_storage_bucket.image_bucket.name}"
+  enable_cdn  = true
+}
+
+resource "google_storage_bucket" "image_bucket" {
+  name     = "image-store-bucket"
+  location = "EU"
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `name` - (Required) The name of the backend bucket.
+
+* `bucket_name` - (Required) The name of the Google Cloud Storage bucket to be used as a backend
+    bucket.
+
+- - -
+
+* `description` - (Optional) The textual description for the backend bucket.
+
+* `enable_cdn` - (Optional) Whether or not to enable the Cloud CDN on the backend bucket.
+
+* `project` - (Optional) The project in which the resource belongs. If it is not provided, the
+    provider project is used.
+
+## Attributes Reference
+
+In addition to the arguments listed above, the following computed attributes are exported:
+
+* `self_link` - The URI of the created resource.