Add Beta support & Beta feature ip_version to google_compute_global_forwarding_rule (hashicorp#265)

* Add versioned Beta support to google_compute_global_forwarding_rule.

* Add Beta support for ip_version in google_compute_global_forwarding_rule.

* Temporary commit with compute_shared_operation.go changes.

* Added a test to see if v1 GFR is still IPV4, moved to a TestCheckFunc

* This API returns the original self links, but let's make sure we don't diff.

Author: rileykarson

docs/r/compute_global_forwarding_rule.html.markdown

@@ -96,6 +96,11 @@ The following arguments are supported:
 * `project` - (Optional) The project in which the resource belongs. If it
     is not provided, the provider project is used.
 
+- - -
+
+* `ip_version` - (Optional, Beta) The IP Version that will be used by this address.
+One of `"IPV4"` or `"IPV6"`.
+
 ## Attributes Reference
 
 In addition to the arguments listed above, the following computed attributes are