fix(nemesis): target node only from data nodes pool

For current moment to support previous behavior of
nemisis, set target node only from data nodes
Also change BaseScyllaCluster.decommission to avoid
operations on zero nodes

sdcm/cluster.py

@@ -4492,7 +4492,7 @@ def cfstat_reached_threshold(self, key, threshold, keyspaces=None):
             keyspaces = self.get_test_keyspaces()
 
         self.log.debug("Waiting for threshold: %s" % (threshold))
-        node = self.nodes[0]
+        node = self.data_nodes[0]
         node_space = 0
         # Calculate space on the disk of all test keyspaces on the one node.
         # It's decided to check the threshold on one node only
@@ -5042,13 +5042,16 @@ def get_node_ip_list(verification_node):
         self.test_config.tester_obj().monitors.reconfigure_scylla_monitoring()
 
     def decommission(self, node: BaseNode, timeout: int | float = None) -> DataCenterTopologyRfControl | None:
-        with node.parent_cluster.cql_connection_patient(node) as session:
-            if tablets_enabled := is_tablets_feature_enabled(session):
-                dc_topology_rf_change = DataCenterTopologyRfControl(target_node=node)
-                dc_topology_rf_change.decrease_keyspaces_rf()
+        if not node._is_zero_token_node:
+            with node.parent_cluster.cql_connection_patient(node) as session:
+                if tablets_enabled := is_tablets_feature_enabled(session):
+                    dc_topology_rf_change = DataCenterTopologyRfControl(target_node=node)
+                    dc_topology_rf_change.decrease_keyspaces_rf()
         with adaptive_timeout(operation=Operations.DECOMMISSION, node=node):
             node.run_nodetool("decommission", timeout=timeout, long_running=True, retry=0)
         self.verify_decommission(node)
+        if node._is_zero_token_node:
+            return None
         return dc_topology_rf_change if tablets_enabled else None
 
     @property