plumb --enable_garbage_collector from environment variable;

adding a simple e2e test

cluster/common.sh

@@ -645,6 +645,7 @@ MANIFEST_URL: $(yaml-quote ${MANIFEST_URL:-})
 MANIFEST_URL_HEADER: $(yaml-quote ${MANIFEST_URL_HEADER:-})
 NUM_NODES: $(yaml-quote ${NUM_NODES})
 INITIAL_ETCD_CLUSTER: $(yaml-quote ${MASTER_NAME})
+ENABLE_GARBAGE_COLLECTOR: $(yaml-quote ${ENABLE_GARBAGE_COLLECTOR:-false})
 EOF
     if [ -n "${APISERVER_TEST_ARGS:-}" ]; then
       cat >>$file <<EOF