Backport gitlab-org/gitlab-ci-yml!128 - Fix kubectl version to 1.8.6

This commit extracts `kubectl`, `helm` and `codeclimate` versions as CI
variables.

`kubectl` changes from latest stable version to `1.8.6`, the other two are just
extracted in order to be easily updated; now we can also test tool upgrades
overriding CI secret variables.

changelogs/unreleased/ac-autodevopfix-kubectl-version.yml

+---
+title: Force Auto DevOps kubectl version to 1.8.6
+merge_request: 16218
+author:
+type: fixed

vendor/gitlab-ci-yml/Auto-DevOps.gitlab-ci.yml

@@ -34,6 +34,10 @@ variables:
   POSTGRES_ENABLED: "true"
   POSTGRES_DB: $CI_ENVIRONMENT_SLUG
 
+  KUBERNETES_VERSION: 1.8.6
+  HELM_VERSION: 2.6.1
+  CODECLIMATE_VERSION: 0.69.0
+
 stages:
   - build
   - test
@@ -250,8 +254,8 @@ production:
              --volume /var/run/docker.sock:/var/run/docker.sock \
              --volume /tmp/cc:/tmp/cc"
 
-    docker run ${cc_opts} codeclimate/codeclimate:0.69.0 init
-    docker run ${cc_opts} codeclimate/codeclimate:0.69.0 analyze -f json > codeclimate.json
+    docker run ${cc_opts} "codeclimate/codeclimate:${CODECLIMATE_VERSION}" init
+    docker run ${cc_opts} "codeclimate/codeclimate:${CODECLIMATE_VERSION}" analyze -f json > codeclimate.json
   }
 
   function sast() {
@@ -323,11 +327,11 @@ production:
     apk add glibc-2.23-r3.apk
     rm glibc-2.23-r3.apk
 
-    curl https://kubernetes-helm.storage.googleapis.com/helm-v2.6.1-linux-amd64.tar.gz | tar zx
+    curl "https://kubernetes-helm.storage.googleapis.com/helm-v${HELM_VERSION}-linux-amd64.tar.gz" | tar zx
     mv linux-amd64/helm /usr/bin/
     helm version --client
 
-    curl -L -o /usr/bin/kubectl https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/linux/amd64/kubectl
+    curl -L -o /usr/bin/kubectl "https://storage.googleapis.com/kubernetes-release/release/v${KUBERNETES_VERSION}/bin/linux/amd64/kubectl"
     chmod +x /usr/bin/kubectl
     kubectl version --client
   }