Add a configuration option (`set_acls`) for Zookeeper DCS so that Kazoo will apply a default ACL for each znode that it creates. The intention is to improve security of the znodes when a single Zookeeper cluster is used as the DCS for multiple Patroni clusters.
Zookeeper [does not apply an ACL to child znodes](https://zookeeper.apache.org/doc/current/zookeeperProgrammers.html#sc_ZooKeeperAccessControl), so permissions can't be set at the `scope` level and then be inherited by other znodes that Patroni creates.
Kazoo instead [provides an option for configuring a default_acl](https://kazoo.readthedocs.io/en/latest/api/client.html#kazoo.client.KazooClient.__init__) that will be applied on node creation.
Example configuration in Patroni might then be:
```
zookeeper:
set_acls:
CN=principal1: [ALL]
CN=principal2:
- READ
```
