use busybox instead of ubuntu for proxy image pulling test

busybox is smaller. gliderlabs/alpine is even smaller, but is used by
weaveexec, so we can't remove it.

test/610_proxy_wait_for_weave_test.sh

@@ -4,7 +4,7 @@
 
 start_suite "Proxy waits for weave to be ready before running container commands"
 weave_on $HOST1 launch-proxy
-BASE_IMAGE=ubuntu
+BASE_IMAGE=busybox
 # Ensure the base image does not exist, so that it will be pulled
 if (docker_on $HOST1 images $BASE_IMAGE | grep $BASE_IMAGE); then
   docker_on $HOST1 rmi $BASE_IMAGE