Bump timeout for subnet delete from 10 to 20 mins

We've seen subnet deletion failures that we think we can trace to ENIs
left behind by NLB. We don't have permission to detach or delete those
ENIs, so our only recourse is to wait longer.

Both the state change timeout and the overall resource timeout is
increased.

aws/resource_aws_subnet.go

@@ -25,7 +25,7 @@ func resourceAwsSubnet() *schema.Resource {
 
 		Timeouts: &schema.ResourceTimeout{
 			Create: schema.DefaultTimeout(10 * time.Minute),
-			Delete: schema.DefaultTimeout(10 * time.Minute),
+			Delete: schema.DefaultTimeout(20 * time.Minute),
 		},
 
 		SchemaVersion: 1,
@@ -320,7 +320,7 @@ func resourceAwsSubnetDelete(d *schema.ResourceData, meta interface{}) error {
 	wait := resource.StateChangeConf{
 		Pending:    []string{"pending"},
 		Target:     []string{"destroyed"},
-		Timeout:    10 * time.Minute,
+		Timeout:    20 * time.Minute,
 		MinTimeout: 1 * time.Second,
 		Refresh: func() (interface{}, string, error) {
 			_, err := conn.DeleteSubnet(req)