Bump helm and kubectl versions

Bump the helm and kubectl used in our Kubernetes integration, used e.g.
to install apps.

Note I have only bumped to the latest patch of the v1.11 series for
kubectl as GKE clusters are still on 1.10/1.11

changelogs/unreleased/gitlab_kubernetes_helm_bump.yml

+---
+title: Bump Helm and kubectl used in Kubernetes integration to 2.12.3 and 1.11.7 respectively
+merge_request: 25268
+author:
+type: other

lib/gitlab/kubernetes/helm.rb

@@ -3,8 +3,8 @@
 module Gitlab
   module Kubernetes
     module Helm
-      HELM_VERSION = '2.12.2'.freeze
-      KUBECTL_VERSION = '1.11.0'.freeze
+      HELM_VERSION = '2.12.3'.freeze
+      KUBECTL_VERSION = '1.11.7'.freeze
       NAMESPACE = 'gitlab-managed-apps'.freeze
       SERVICE_ACCOUNT = 'tiller'.freeze
       CLUSTER_ROLE_BINDING = 'tiller-admin'.freeze

spec/lib/gitlab/kubernetes/helm/pod_spec.rb

@@ -30,7 +30,7 @@ describe Gitlab::Kubernetes::Helm::Pod do
       it 'should generate the appropriate specifications for the container' do
         container = subject.generate.spec.containers.first
         expect(container.name).to eq('helm')
-        expect(container.image).to eq('registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/2.12.2-kube-1.11.0')
+        expect(container.image).to eq('registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/2.12.3-kube-1.11.7')
         expect(container.env.count).to eq(3)
         expect(container.env.map(&:name)).to match_array([:HELM_VERSION, :TILLER_NAMESPACE, :COMMAND_SCRIPT])
         expect(container.command).to match_array(["/bin/sh"])