Reset *RemovalPoliciesMode for node delete

'update' *RemovalPoliciesMode during 'node delete' can result in
undesired behaviour as we flush the blacklist history and use
the freshly constructed blacklist and decrease the node count.

Change-Id: Iebd0089b6ab35c84e4f20e83f1990c1c5568017f
Closes-Bug: #1848651

tripleo_common/actions/scale.py

@@ -118,6 +118,11 @@ class ScaleDownAction(base.TripleOAction):
                 'resource_list': [r.resource_name for r in role_resources]
             }]
 
+            # force reset the removal_policies_mode to 'append'
+            # as 'update' can lead to deletion of unintended nodes.
+            removal_mode = "{0}RemovalPoliciesMode".format(role)
+            stack_params[removal_mode] = 'append'
+
         return stack_params
 
     def run(self, context):

tripleo_common/tests/actions/test_scale.py

@@ -140,7 +140,8 @@ class ScaleDownActionTest(base.TestCase):
             template={'heat_template_version': '2016-04-30'}
         )
 
-        clear_list = list(['ComputeCount', 'ComputeRemovalPolicies'])
+        clear_list = list(['ComputeCount', 'ComputeRemovalPolicies',
+                           'ComputeRemovalPoliciesMode'])
         _, kwargs = heatclient.stacks.update.call_args
         self.assertEqual(set(kwargs['clear_parameters']), set(clear_list))
         self.assertEqual(kwargs['environment'], env)