Change docker health checker to using `docker ps`

cluster/gce/trusty/node.yaml

@@ -258,7 +258,7 @@ script
 	# We simply kill the process when there is a failure. Another upstart job will automatically
 	# restart the process.
 	while [ 1 ]; do
-		if ! timeout 10 docker version > /dev/null; then
+		if ! timeout 20 docker ps > /dev/null; then
 			echo "Docker daemon failed!"
 			pkill docker
 		fi

cluster/saltbase/salt/supervisor/docker-checker.sh

@@ -25,7 +25,7 @@ echo "waiting a minute for startup"
 sleep 60
 
 while true; do
-  if ! sudo timeout 10 docker version > /dev/null; then
+  if ! sudo timeout 20 docker ps > /dev/null; then
     echo "Docker failed!"
     exit 2
   fi