Update Kubernetes from v1.15.1 to v1.15.2

* https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.15.md#v1152

google-cloud/container-linux/kubernetes/README.md

@@ -11,7 +11,7 @@ Typhoon distributes upstream Kubernetes, architectural conventions, and cluster
 
 ## Features <a href="https://www.cncf.io/certification/software-conformance/"><img align="right" src="https://storage.googleapis.com/poseidon/certified-kubernetes.png"></a>
 
-* Kubernetes v1.15.1 (upstream, via [kubernetes-incubator/bootkube](https://github.com/kubernetes-incubator/bootkube))
+* Kubernetes v1.15.2 (upstream, via [kubernetes-incubator/bootkube](https://github.com/kubernetes-incubator/bootkube))
 * Single or multi-master, [Calico](https://www.projectcalico.org/) or [flannel](https://github.com/coreos/flannel) networking
 * On-cluster etcd with TLS, [RBAC](https://kubernetes.io/docs/admin/authorization/rbac/)-enabled, [network policy](https://kubernetes.io/docs/concepts/services-networking/network-policies/)
 * Advanced features like [worker pools](https://typhoon.psdn.io/advanced/worker-pools/), [preemptible](https://typhoon.psdn.io/cl/google-cloud/#preemption) workers, and [snippets](https://typhoon.psdn.io/advanced/customization/#container-linux) customization

google-cloud/container-linux/kubernetes/bootkube.tf

@@ -1,6 +1,6 @@
 # Self-hosted Kubernetes assets (kubeconfig, manifests)
 module "bootkube" {
-  source = "git::https://github.com/poseidon/terraform-render-bootkube.git?ref=83dd5a7cfc5daa9afc259fd4c3ceb22dec9437da"
+  source = "git::https://github.com/poseidon/terraform-render-bootkube.git?ref=c21da0224984493e92dd2dc7bb3b755c564852fc"
 
   cluster_name          = var.cluster_name
   api_servers           = [format("%s.%s", var.cluster_name, var.dns_zone)]

google-cloud/container-linux/kubernetes/cl/controller.yaml.tmpl

@@ -124,7 +124,7 @@ storage:
       contents:
         inline: |
           KUBELET_IMAGE_URL=docker://k8s.gcr.io/hyperkube
-          KUBELET_IMAGE_TAG=v1.15.1
+          KUBELET_IMAGE_TAG=v1.15.2
     - path: /etc/sysctl.d/max-user-watches.conf
       filesystem: root
       contents:

google-cloud/container-linux/kubernetes/workers/cl/worker.yaml.tmpl

@@ -94,7 +94,7 @@ storage:
       contents:
         inline: |
           KUBELET_IMAGE_URL=docker://k8s.gcr.io/hyperkube
-          KUBELET_IMAGE_TAG=v1.15.1
+          KUBELET_IMAGE_TAG=v1.15.2
     - path: /etc/sysctl.d/max-user-watches.conf
       filesystem: root
       contents:
@@ -112,7 +112,7 @@ storage:
             --volume config,kind=host,source=/etc/kubernetes \
             --mount volume=config,target=/etc/kubernetes \
             --insecure-options=image \
-            docker://k8s.gcr.io/hyperkube:v1.15.1 \
+            docker://k8s.gcr.io/hyperkube:v1.15.2 \
             --net=host \
             --dns=host \
             --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)