fix(perf): update ES with append_scylla_yaml dict

Since `append_scylla_yaml` is a dict and has value set, updating ES
fails with `failed to parse field [setup_details.append_scylla_yaml] of
type [text]` error.

Fix by converting it to `str` when generating `setup_details`.

sdcm/db_stats.py

@@ -638,6 +638,7 @@ def get_setup_details(self):
 
         setup_details['db_cluster_node_details'] = {}
         setup_details['sysctl_output'] = []
+        setup_details['append_scylla_yaml'] = str(self.params.get('append_scylla_yaml') or '')
         return setup_details
 
     def get_test_details(self):

unit_tests/test_scylla_yaml.py

@@ -398,7 +398,8 @@ def test_scylla_yaml(self):
                 'virtual_dirty_soft_limit': None,
                 'volatile_system_keyspace_for_testing': None,
                 'workdir': None,
-                'write_request_timeout_in_ms': None
+                'write_request_timeout_in_ms': None,
+                'enable_tablets': None,
             }
         )