Add Beta support & Beta feature ip_version to google_compute_global_address (hashicorp#250)

* Make google_compute_global_address a versioned resource with Beta support.

* Added Beta support for ip_version in google_compute_global_address.

* Move checks to TestCheckFuncs, add a regression test for IPV4 on v1 resources.

* Consolidated TestCheckFuncs to a single function.

* Add missing return statement.

* Fix IPV4 test

* Clarified comment.

Author: rileykarson

docs/r/compute_global_address.html.markdown

@@ -33,6 +33,11 @@ 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