Make the ZooKeeper config customizable in Accumulo Mini Cluster

Currently there is only a hardcoded list of zookeeper properties when MiniAccumuloCluster starts it's own zookeeper. It would be nice if we could define our custom Zookeeper config properties that would extend and if necessary override the hardcoded ones. Maybe something similar to the current siteConfig?

Start by reading MiniAccumuloCluster startup handling for the hardcoded ZooKeeper properties and compare it with the existing siteConfig customization. Determine how custom properties should extend and override the defaults, then verify that a MiniAccumuloCluster can start with both default and user-supplied ZooKeeper settings.

Written by the indexing model from the issue text.