Fix issues with substr for Service Accounts

[lawliet89]

Fixes the following error:

* module.gke.google_service_account.cluster_service_account: substr: 'offset + length' cannot be larger than the length of the string in:

tf-gke-${substr(var.name, 0, 20)}

[morgante]

LGTM, can merge once CI is green.

[adrienthebo]

We'll need to merge #87 before CI will pass on this PR; hopefully we can wrap that up by EOD.

[adrienthebo]

Verified with the following diff:

diff --git i/examples/simple_zonal/main.tf w/examples/simple_zonal/main.tf
index a0dc9ff..c95038a 100644
--- i/examples/simple_zonal/main.tf
+++ w/examples/simple_zonal/main.tf
@@ -27,7 +27,7 @@ provider "google" {
 module "gke" {
   source            = "../../"
   project_id        = "${var.project_id}"
-  name              = "${local.cluster_type}-cluster${var.cluster_name_suffix}"
+  name              = "sz"
   regional          = false
   region            = "${var.region}"
   zones             = "${var.zones}"

I'll submit a followup PR to add this code path to our test coverage.