Add option to disable ipv6 dns lookup

New variable disable_ipv6_dns in kubernetes/preinstall.

roles/kubernetes/preinstall/defaults/main.yml

@@ -29,6 +29,10 @@ common_required_pkgs:
   - rsync
   - bash-completion
 
+# Set to true if your network does not support IPv6
+# This maybe necessary for pulling Docker images from
+# GCE docker repository
+disable_ipv6_dns: false
 
 
 # For the openstack integration kubelet will need credentials to access