Fix malformed mistal_executor kolla config file

In the event UndercloudConfigFilePath is empty, the produced
/var/lib/kolla/config_files/mistral_executor.json file was malformed
and kolla would complain with:

+ sudo -E kolla_set_configs
INFO:__main__:Loading config file at /var/lib/kolla/config_files/config.json
INFO:__main__:Validating config file
ERROR:__main__:Unexpected error:
Traceback (most recent call last):
  File "/usr/local/bin/kolla_set_configs", line 412, in main
    config = load_config()
  File "/usr/local/bin/kolla_set_configs", line 297, in load_config
    validate_config(config)
  File "/usr/local/bin/kolla_set_configs", line 238, in validate_config
    if not set(data.keys()) >= required_keys:
AttributeError: 'unicode' object has no attribute 'keys'

Change-Id: Ibb7ed9e63404729ab8c2406d7e3c4733f465ea98
Closes-Bug: #1816441

ci/environments/scenario009-multinode.yaml

@@ -6,7 +6,6 @@ resource_registry:
   OS::TripleO::Services::OpenShift::Worker: ../../extraconfig/services/openshift-worker.yaml
   OS::TripleO::Services::OpenShift::Infra: ../../extraconfig/services/openshift-infra.yaml
 
-
 parameter_defaults:
   ControllerServices:
     - OS::TripleO::Services::DisableUnbound

deployment/mistral/mistral-executor-container-puppet.yaml

@@ -134,17 +134,18 @@ outputs:
         /var/lib/kolla/config_files/mistral_executor.json:
           command: /usr/bin/mistral-server --config-file=/etc/mistral/mistral.conf --log-file=/var/log/mistral/executor.log --server=executor
           config_files:
-            - source: "/var/lib/kolla/config_files/src/*"
-              dest: "/"
-              merge: true
-              preserve_properties: true
-            - if:
-              - undercloud_config_file_path_unset
-              - ''
-              - source: '/var/lib/undercloud.conf'
-                dest: '/var/lib/mistral/undercloud.conf'
-                merge: true
-                preserve_properties: true
+            list_concat:
+              - - source: "/var/lib/kolla/config_files/src/*"
+                  dest: "/"
+                  merge: true
+                  preserve_properties: true
+              - if:
+                - undercloud_config_file_path_unset
+                - null
+                - - source: '/var/lib/undercloud.conf'
+                    dest: '/var/lib/mistral/undercloud.conf'
+                    merge: true
+                    preserve_properties: true
           permissions:
             - path: /var/log/mistral
               owner: mistral:mistral