Reschedule during cold migration fails if compute RPC is pinned to 5.0

[Balazs Gibizer]

Steps to reproduce
==================

1) Set the [upgrade_levels]/compute config parameter to '5.0'
2) Boot a simple instance
3) Prepare your env that the first destination host for the next migration will fail during _prep_resize (for example the resize_claim fails)
4) Do the migration

Expected result
===============

The migration is re-scheduled to an alternative host where it succeeds.

Actual result
=============

The migration fails, the instance goes to ERROR state, and the log contains a nice stack trace:

    2019-09-20 08:15:26,980 ERROR [nova.compute.manager] Error trying to reschedule
    Traceback (most recent call last):
      File "nova/compute/manager.py", line 4588, in _reschedule_resize_or_reraise
        request_spec=request_spec, host_list=host_list)
      File "nova/conductor/api.py", line 96, in resize_instance
        request_spec=request_spec, host_list=host_list)
      File "nova/conductor/rpcapi.py", line 338, in migrate_server
        return cctxt.call(context, 'migrate_server', **kw)
      File "/home/opnfv/rtox/nova/functional/.tox/functional/local/lib/python2.7/site-packages/oslo_messaging/rpc/client.py", line 181, in call
        transport_options=self.transport_options)
      File "/home/opnfv/rtox/nova/functional/.tox/functional/local/lib/python2.7/site-packages/oslo_messaging/transport.py", line 129, in _send
        transport_options=transport_options)
      File "/home/opnfv/rtox/nova/functional/.tox/functional/local/lib/python2.7/site-packages/oslo_messaging/_drivers/impl_fake.py", line 224, in send
        transport_options)
      File "/home/opnfv/rtox/nova/functional/.tox/functional/local/lib/python2.7/site-packages/oslo_messaging/_drivers/impl_fake.py", line 210, in _send
        raise failure
    AttributeError: 'dict' object has no attribute 'flavor'

Reproduction in functional test env coming soon.

Environment
===========

nova commit: fb3ba44438546835c29d23492cf8604df153f3c2

[Matt Riedemann (mriedem)]

I believe I already have a fix for this: https://review.opendev.org/#/c/680762/