Copy kubectl from docker container

Nearly the last stage of source all components to containers.
Kubectl will be called from hyperkube image.

Remaining tasks:
 * Move kube_version variable to kubernetes/preinstall
 * Drop placeholder download.nothing requirement

roles/kubernetes/master/meta/main.yml

@@ -1,6 +1,6 @@
 ---
 dependencies:
-  - role: download
-    file: "{{ downloads.kubernetes_kubectl }}"
+  - role: download # For kube_version variable
+    file: "{{ downloads.nothing }}"
   - { role: etcd }
   - { role: kubernetes/node }