Update zktools/Waltz to assign server partition ownership

[whynick1]

zktools needs to be updated to add a new CLI command to set server:partition assignments. Currently, the PartitionAssignmentPolicy dictates what the partition assignments for each server are. We need to add a way to force the assignment in a specific way.

This ticket also requires us to add a new command in ZooKeeperCli in Waltz. The new command should be:

assign --partition=[partition_id] --server=[server_id]
unassign --partition=[partition_id] --server=[server_id]