Allow setting ZooKeeperCluster's size via ZK_CLUSTER_SIZE
Allow setting ZooKeeperCluster's size via ZK_CLUSTER_SIZE

Signed-off-by: Raul Gutierrez S <f25f6873bbbde69f1fe653b3e6bd40d543b8d0e0@itevenworks.net>

bsanders/kazoo,python-zk/kazoo,bsanders/kazoo,python-zk/kazoo