refactor(cluster.py): extract node health checks
refactor(cluster.py): extract node health checks

Move node health checks out of the loop inside check_node_health()
to be able to add another checks in derived classes.

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