Don't persist RequestSpec.requested_destination

The RequestSpec.requested_destination, similar to the
retry field, is per-request/operation, and persisting
it can caues issues when subsequent move requests.

For example, if you cold migrate a server to a specific
host and then live migrate that server without specifying
a host, the requested target host from the cold migrate
is sent to the scheduler for the live migration, but since
that is where the instance is already running, it's
rejected with NoValidHost.

This is a similar issue to the need to call
RequestSpec.reset_forced_destinations() in all move operations
in conductor. However, rather than try to whack this mole in
every place the request spec is sent to the scheduler, like
reset_forced_destinations() is used, we simply don't need to
persist the requested_destination field since it's just a
vehicle to tell the scheduler which host we want.

The related functional regression test is updated to show
the bug is now fixed.

Conflicts:
      nova/objects/request_spec.py

NOTE(mriedem): The conflict is due to not having change
Icb295bbd8c83e2e340a7ac3ecc1f159e0db7c7b1 in Queens.

Change-Id: I2a78f0754c63381c57e7e1c610d0938b6df0f537
Closes-Bug: #1797580
(cherry picked from commit ce3af5e33a)
(cherry picked from commit 954b2004a1)

nova/objects/request_spec.py

@@ -511,9 +511,11 @@ class RequestSpec(base.NovaObject):
             if 'instance_group' in spec and spec.instance_group:
                 spec.instance_group.members = None
                 spec.instance_group.hosts = None
-            # NOTE(mriedem): Don't persist retries since those are per-request
-            if 'retry' in spec and spec.retry:
-                spec.retry = None
+            # NOTE(mriedem): Don't persist retries or requested_destination
+            # since those are per-request
+            for excluded in ('retry', 'requested_destination'):
+                if excluded in spec and getattr(spec, excluded):
+                    setattr(spec, excluded, None)
 
             db_updates = {'spec': jsonutils.dumps(spec.obj_to_primitive())}
             if 'instance_uuid' in updates: