Make partition processor placements configurable

[tillrohrmann]

To control how partition processors are placed, we need to make the placement configurable. The configuration could include the overall partition processor placement strategy (ReplicationStrategy::OnAllNodes, ReplicationStrategy::Factor) or a specific set of worker nodes. Additionally, it should allow to configure a specific worker node to be the leader. Based on this configuration, the Scheduler should accept all equivalent placements as acceptable.

Additionally, we need an API to modify this configuration (e.g. by storing it in the metadata store so that all nodes read it eventually).