provider/openstack: use '4' as default for ip_version of subnet

builtin/providers/openstack/resource_openstack_networking_subnet_v2.go

@@ -69,7 +69,8 @@ func resourceNetworkingSubnetV2() *schema.Resource {
 			},
 			"ip_version": &schema.Schema{
 				Type:     schema.TypeInt,
-				Required: true,
+				Optional: true,
+				Default:  4,
 				ForceNew: true,
 			},
 			"enable_dhcp": &schema.Schema{

builtin/providers/openstack/resource_openstack_networking_subnet_v2_test.go

@@ -99,7 +99,6 @@ var testAccNetworkingV2Subnet_basic = fmt.Sprintf(`
     region = "%s"
     network_id = "${openstack_networking_network_v2.network_1.id}"
     cidr = "192.168.199.0/24"
-    ip_version = 4
   }`, OS_REGION_NAME, OS_REGION_NAME)
 
 var testAccNetworkingV2Subnet_update = fmt.Sprintf(`
@@ -114,6 +113,5 @@ var testAccNetworkingV2Subnet_update = fmt.Sprintf(`
     name = "tf-test-subnet"
     network_id = "${openstack_networking_network_v2.network_1.id}"
     cidr = "192.168.199.0/24"
-    ip_version = 4
     gateway_ip = "192.168.199.1"
   }`, OS_REGION_NAME, OS_REGION_NAME)

website/source/docs/providers/openstack/r/networking_subnet_v2.html.markdown

@@ -21,7 +21,6 @@ resource "openstack_networking_network_v2" "network_1" {
 resource "openstack_networking_subnet_v2" "subnet_1" {
   network_id = "${openstack_networking_network_v2.network_1.id}"
   cidr = "192.168.199.0/24"
-  ip_version = 4
 }
 ```
 
@@ -40,7 +39,7 @@ The following arguments are supported:
 * `cidr` - (Required) CIDR representing IP range for this subnet, based on IP
     version. Changing this creates a new subnet.
 
-* `ip_version` - (Required) IP version, either 4 or 6. Changing this creates a
+* `ip_version` - (Optional) IP version, either 4 (default) or 6. Changing this creates a
     new subnet.
 
 * `name` - (Optional) The name of the subnet. Changing this updates the name of