Fixing aws_elb schema

If not suppling the `availability_zones`, they will be computed
(meaning an update/refresh will retrieve the info and update the values
to the state file).

So without the `Computed = true` the diff will always flag this as a
change, even when it’s not.

builtin/providers/aws/resource_aws_elb.go

@@ -41,6 +41,7 @@ func resourceAwsElb() *schema.Resource {
 				Elem:     &schema.Schema{Type: schema.TypeString},
 				Optional: true,
 				ForceNew: true,
+				Computed: true,
 				Set: func(v interface{}) int {
 					return hashcode.String(v.(string))
 				},