Bump GKE min_master_version, lock major and minor only

Author: pst

quickstart/src/configurations/gke/config.auto.tfvars

@@ -18,7 +18,7 @@ clusters = {
       base_domain = ""
 
       # Initial desired K8s version, will be upgraded automatically
-      cluster_min_master_version = "1.13.6"
+      cluster_min_master_version = "1.15"
 
       # Initial number of desired nodes per zone
       cluster_initial_node_count = 1

tests/config.auto.tfvars

@@ -24,7 +24,7 @@ clusters = {
       project_id                 = "terraform-kubestack-testing"
       name_prefix                = "kbstacctest"
       base_domain                = "infra.serverwolken.de"
-      cluster_min_master_version = "1.13"
+      cluster_min_master_version = "1.15"
       cluster_min_node_count     = 1
       cluster_max_node_count     = 1
       region                     = "europe-west1"