Add support to be able to specify a completely custom ClusterManager Implementation

[bigsur0]

Kindly loosen restrictions on ClusterManager configuration such that a custom ClusterManager implementation can be provided via: rubix.custom.clustermanager.class=org.foo.bar.MyClusterManager

Please see #154 which gives the impression of broader pluggability, but is only relevant to plugging in one of the well known ClusterManager implementations provided in Rubix.