feature(alternator): add on more nodes to the 3h sanity
feature(alternator): add on more nodes to the 3h sanity

* since we are now using nemsis, we need more nodes for that.
  we forgot adding that, and it's causing cluster to be non functional
  once we remove one node (the alternator table RF=3)

* enabled mgmt
* lowered the load a bit, so we won't be maxed out
* disabled nemesis during prepare

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests