fix(k8s): remove default fallback to loader based on scylla image

now that images doesn't have c-s installed in them, we shouldn't
be defaulting to scylla images anymore.

sdcm/cluster_k8s/__init__.py

@@ -3098,10 +3098,7 @@ def node_startup(self, node: BasePodContainer, verbose: bool = False, **kwargs)
     def _get_docker_image(self):
         if loader_image := self.params.get('stress_image.cassandra-stress'):
             return loader_image
-        else:
-            docker_image = self.params.get('docker_image')
-            scylla_version = self.params.get('scylla_version')
-            return f"{docker_image}:{scylla_version}"
+        raise ValueError("No 'stress_image.cassandra-stress' option found in the test configuration")
 
     def add_nodes(self,
                   count: int,

sdcm/remote/kubernetes_cmd_runner.py

@@ -396,7 +396,7 @@ def _get_docker_image(self, command) -> str:
             return params.get("stress_image.ycsb")
         if loader_image := params.get('stress_image.cassandra-stress'):
             return loader_image
-        return f"{params.get('docker_image')}:{params.get('scylla_version')}"
+        raise ValueError("No loader image found in the params")
 
     def _get_pod_status(self) -> dict:
         result_raw = self.context.config.k8s_kluster.kubectl(