Comment 0 for bug 1841904

When confirming or reverting a VERIFY_RESIZE instance, nova does't clear
requested_destinatoin and ignore_hosts of instance's RequestSpec. It will
cause the failure of rebuilding instance, because requested_destination
and ignore_hosts affect the scheduling in rebuilding process.

We have two compute nodes, let's call them hostA and hostB, and we have an
instance built on hostA, then:

1. migrate instance from hostA to hostB, and wait for instance status changing
to VERIFY_RESIZE. Now instance RequestSpec's requested_destination is hostB,
ignore_hosts equals ['hostA'] as default CONF.allow_resize_to_same_host is False.

2. confirming or reverting the resize does nothing about instance RequestSpec.

3. rebuild instance with new image, nova set request_spec.force_hosts and
request_spec.force_nodes with current hostA [1]. Scheduler get hostB as matching
one, and striping hostA as ignoring one. Then no force host or force node matched,
NoValidHost expcetion is raised [2].

[1].https://github.com/openstack/nova/blob/eb6fcb21917d56e4e287513c1159ad4656382714/nova/compute/api.py#L3362
[2].https://github.com/openstack/nova/blob/eb6fcb21917d56e4e287513c1159ad4656382714/nova/scheduler/host_manager.py#L569-L588