Make Cassandra replication factor configurable

It is useful to be able to configure the replication factor for the Hawkular-metrics Cassandra keyspace, such that it is not necessary to start Hawkular, manually alter the replication factor and run a repair when setting up a new cluster.

A simple numeric setting is fine for my uses cases (using SimpleStrategy), but allowing configuration of the strategy and it's parameters would be the most generic fix.