Added integration load test for NetworkTopologyStrategy

I added an integration test for the NetworkTopologyStrategy based on the normal load test. Since it's not directly supported (as far as I can see) by cassandra-unit to start different cassandra configs in one jvm I changed the cassandra config for all tests to use a PropertyFileSnitch which configures a datacenter and rack. Imo this should be ok because the replication strategy is based on the schema and we don't want to test the cassandra config in general which means that SimpleStrategy tests should not be affected in a negative way. 

I also upgraded the cassandra-unit to the next version because it uses cassandra 2.1 instead of cassandra 2.0.5

Please let me know if this is fine for you otherwise it should be possible to hack around cassandra-unit and run it with different configs but that might take some more time.

Best
Martin