Set kube_image_repo use {{ gcr_image_repo }} (#5314)

To aviod repeat "gcr.io" again.

inventory/sample/group_vars/k8s-cluster/k8s-cluster.yml

@@ -23,7 +23,7 @@ kube_api_anonymous_auth: true
 kube_version: v1.16.3
 
 # kubernetes image repo define
-kube_image_repo: "gcr.io/google-containers"
+kube_image_repo: "{{ gcr_image_repo }}/google-containers"
 
 # Where the binaries will be downloaded.
 # Note: ensure that you've enough disk space (about 1G)