Handle missing dst_node parameter in zone_migration

For compute nodes, nova works fine if a destination node is not
specified, so this change makes sure we're not passing None when the
user does not set one to avoid an error.

Partial-Bug: 2108988

Change-Id: Ida1f18b97697c041819e29f935aa5e232848226a

releasenotes/notes/zone-migration-missing-dst-node-bd0377af1f1ed245.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    The zone migration strategy no longer requires a dst_node to be passed.
+    When unspecified, the Nova scheduler will select an appropriate host automatically.
+    This brings the implementation of the strategy in line with the the api schema
+    where dest_node is optional.
+
+    See: https://bugs.launchpad.net/watcher/+bug/2108988 for more details.

watcher/decision_engine/strategy/strategies/zone_migration.py

@@ -444,9 +444,12 @@ class ZoneMigration(base.ZoneMigrationBaseStrategy):
 
     def _live_migration(self, instance, src_node, dst_node):
         parameters = {"migration_type": "live",
-                      "destination_node": dst_node,
                       "source_node": src_node,
                       "resource_name": instance.name}
+        if dst_node:
+            # if dst_node is None, do not add it to the parameters for the
+            # migration action, and let Nova figure out the destination node
+            parameters["destination_node"] = dst_node
         self.solution.add_action(
             action_type="migrate",
             resource_id=instance.id,
@@ -455,9 +458,12 @@ class ZoneMigration(base.ZoneMigrationBaseStrategy):
 
     def _cold_migration(self, instance, src_node, dst_node):
         parameters = {"migration_type": "cold",
-                      "destination_node": dst_node,
                       "source_node": src_node,
                       "resource_name": instance.name}
+        if dst_node:
+            # if dst_node is None, do not add it to the parameters for the
+            # migration action, and let Nova figure out the destination node
+            parameters["destination_node"] = dst_node
         self.solution.add_action(
             action_type="migrate",
             resource_id=instance.id,

watcher/tests/decision_engine/strategy/strategies/test_zone_migration.py

@@ -259,10 +259,7 @@ class TestZoneMigration(TestBaseStrategy):
         migration_params = solution.actions[0]['input_parameters']
         # since we have not passed 'dst_node' in the input, we should not have
         # a destination_node in the generated migration action
-        # self.assertNotIn('destination_node', migration_params)
-        # temporarily make the test pass, delete and use the above assert in
-        # followup
-        self.assertIsNone(migration_params['destination_node'])
+        self.assertNotIn('destination_node', migration_params)
 
     def test_execute_cold_migrate_instance(self):
         instance_on_src1 = self.fake_instance(