Multi-cluster tests on real deployments and in Simulator

The test server must orchestrate multiple clusters in order to test multi-cluster replication solutions. In addition, the Simulator must also be able to simulate multiple clusters. This is required for [SnowCannon](https://github.com/apple/foundationdb/issues/1272) and is necessary for testing DR solutions as well.

This involves work in tester interfaces and WorkloadContext to track multiple cluster files and run the workload on each cluster, a new run option, and some changes in SimulatedCluster.