This is interesting. My understanding is resize to different host. The host here means ESXi Host. I am not certain if current openstack support resize across Clusters. 

Taking look at the code. "host_ref = self._get_host_ref_from_name(dest) ", And I think cluster does not fit into this category. 

def migrate_disk_and_power_off(self, context, instance, dest,
                                   flavor):
        """
        Transfers the disk of a running instance in multiple phases, turning
        off the instance before the end.
        """
        # 0. Zero out the progress to begin
        self._update_instance_progress(context, instance,
                                       step=0,
                                       total_steps=RESIZE_TOTAL_STEPS)

        vm_ref = vm_util.get_vm_ref(self._session, instance)
        # Read the host_ref for the destination. If this is None then the
        # VC will decide on placement
        host_ref = self._get_host_ref_from_name(dest)

        # 1. Power off the instance
        self.power_off(instance)
        self._update_instance_progress(context, instance,
                                       step=1,
                                       total_steps=RESIZE_TOTAL_STEPS)

        # 2. Rename the original VM with suffix '-orig'
        name_label = self._get_orig_vm_name_label(instance)
        LOG.debug(_("Renaming the VM to %s") % name_label,
                  instance=instance)
        rename_task = self._session._call_method(
                            self._session._get_vim(),
                            "Rename_Task", vm_ref, newName=name_label)
        self._session._wait_for_task(instance['uuid'], rename_task)
        LOG.debug(_("Renamed the VM to %s") % name_label,
                  instance=instance)
        self._update_instance_progress(context, instance,
                                       step=2,
                                       total_steps=RESIZE_TOTAL_STEPS)

        # Get the clone vm spec
        ds_ref = vm_util.get_datastore_ref_and_name(
                            self._session, self._cluster, host_ref,
                            datastore_regex=self._datastore_regex)[0]
        client_factory = self._session._get_vim().client.factory
        rel_spec = vm_util.relocate_vm_spec(client_factory, ds_ref, host_ref)
        clone_spec = vm_util.clone_vm_spec(client_factory, rel_spec)
        vm_folder_ref = self._get_vmfolder_ref()

        # 3. Clone VM on ESX host
        LOG.debug(_("Cloning VM to host %s") % dest, instance=instance)
        vm_clone_task = self._session._call_method(
                                self._session._get_vim(),
                                "CloneVM_Task", vm_ref,
                                folder=vm_folder_ref,
                                name=instance['uuid'],
                                spec=clone_spec)
        self._session._wait_for_task(instance['uuid'], vm_clone_task)
        LOG.debug(_("Cloned VM to host %s") % dest, instance=instance)
        self._update_instance_progress(context, instance,
                                       step=3,
                                       total_steps=RESIZE_TOTAL_STEPS)