Support config image repository for kubeadm

In some places of which network environment was limited, kubeadm
can't pull images from k8s.gcr.io. This patch add a variable
`KUBEADMIN_IMAGE_REPOSITORY` in order to the developer who located in
these places can set the kubeadm to pull container images from
repository that they can access.

Change-Id: I14aed50077ef0760635e575770fd2274cb759c53

devstack/lib/k8s

@@ -63,6 +63,7 @@ function kubeadm_init {
     cat <<EOF | tee $kubeadm_config_file >/dev/null
 apiVersion: kubeadm.k8s.io/v1beta1
 kind: ClusterConfiguration
+imageRepository: "${KUBEADMIN_IMAGE_REPOSITORY}"
 etcd:
   external:
     endpoints:
@@ -84,6 +85,7 @@ apiVersion: kubelet.config.k8s.io/v1beta1
 kind: KubeletConfiguration
 failSwapOn: false
 EOF
+    sudo kubeadm config images pull --image-repository=${KUBEADMIN_IMAGE_REPOSITORY}
     sudo kubeadm init --config $kubeadm_config_file --ignore-preflight-errors Swap
 
     local kube_config_file=$HOME/.kube/config

devstack/settings

@@ -24,3 +24,6 @@ if [[ ,${ENABLED_SERVICES} =~ ,"k8s-master" ]]; then
     enable_service kube-scheduler
     enable_service kube-proxy
 fi
+
+# Customize kubeadm container images repository
+KUBEADMIN_IMAGE_REPOSITORY=${KUBEADMIN_IMAGE_REPOSITORY:-"k8s.gcr.io"}