providers/google: support optionial uuid naming for Instance Template (#6604)

Auto-generating an Instance Template name (or just its suffix) allows the
create_before_destroy lifecycle option to function correctly on the
Instance Template resource. This in turn allows Instance Group Managers
to be updated without being destroyed.

r/compute_instance_template.html.markdown

@@ -58,6 +58,51 @@ resource "google_compute_instance_template" "foobar" {
 }
 ```
 
+## Using with Instance Group Manager
+
+Instance Templates cannot be updated after creation with the Google
+Cloud Platform API. In order to update an Instance Template, Terraform will
+destroy the existing resource and create a replacement. In order to effectively
+use an Instance Template resource with an [Instance Group Manager resource][1],
+it's recommended to specify `create_before_destroy` in a [lifecycle][2] block.
+Either omit the Instance Template `name` attribute, or specify a partial name
+with `name_prefix`.  Example:
+
+```
+resource "google_compute_instance_template" "instance_template" {
+    name_prefix = "instance-template-"
+    machine_type = "n1-standard-1"
+    region = "us-central1"
+
+    // boot disk
+    disk {
+      ...
+    }
+
+    // networking
+    network_interface {
+      ...
+    }
+ 
+    lifecycle {
+        create_before_destroy = true
+    }
+}
+
+resource "google_compute_instance_group_manager" "instance_group_manager" {
+    name = "instance-group-manager"
+    instance_template = "${google_compute_instance_template.instance_template.self_link}"
+    base_instance_name = "instance-group-manager"
+    zone = "us-central1-f"
+    target_size = "1"
+}
+```
+
+With this setup Terraform generates a unique name for your Instance
+Template and can then update the Instance Group manager without conflict before
+destroying the previous Instance Template.
+
+
 ## Argument Reference
 
 Note that changing any field for this resource forces a new resource to be created.
@@ -70,9 +115,12 @@ The following arguments are supported:
 
 * `machine_type` - (Required) The machine type to create.
 
-* `name` - (Required) A unique name for the resource, required by GCE.
-
 - - -
+* `name` - (Optional) The name of the instance template. If you leave
+  this blank, Terraform will auto-generate a unique name.
+
+* `name_prefix` - (Optional) Creates a unique name beginning with the specified
+  prefix. Conflicts with `name`.
 
 * `can_ip_forward` - (Optional) Whether to allow sending and receiving of
     packets with non-matching source or destination IPs. This defaults to false.
@@ -191,3 +239,6 @@ exported:
 * `self_link` - The URI of the created resource.
 
 * `tags_fingerprint` - The unique fingerprint of the tags.
+
+[1]: /docs/providers/google/r/compute_instance_group_manager.html
+[2]: /docs/configuration/resources.html#lifecycle