instance path not be deleted after evacuate

[Xiao Gong]

Description
===========
After evacuate an instance to a new host, then live-migration it to the old host, nova-compute report "DestinationDiskExists" error.

When nova-compute on old host restarts, it only deletes the evacuated instance path that needs destroying disks. If the instance is on shared block storage such as ceph, nova-compute will not delete instance path.

Steps to reproduce
===========
* create an instance on a compute node using ceph backend
* crash the compute node
* evacuate the instance
* reboot the crashed node

Expected result
===============
The evacuated instance path should be deleted.

Actual result
=============
After crashed nova-compute restarts, the instance path still exists.

Environment
===========
I used stable Newton. But I think it's still valid on the master.

[Sean Dague (sdague)]

Can we get a formal verification on master. So much of the ceph code has changed since then.

[Xiao Gong (gongxiao)]

I only have an Openstack in Newton, and I verified this bug on it.
I compare the code between Newton and master, there is no decisive difference.

relative code about cleanup method in master:

        is_shared_block_storage = False
        if migrate_data and 'is_shared_block_storage' in migrate_data:
            is_shared_block_storage = migrate_data.is_shared_block_storage
        if destroy_disks or is_shared_block_storage:
            attempts = int(instance.system_metadata.get('clean_attempts',
                                                        '0'))
            success = self.delete_instance_files(instance)
            # NOTE(mriedem): This is used in the _run_pending_deletes periodic
            # task in the compute manager. The tight coupling is not great...
            instance.system_metadata['clean_attempts'] = str(attempts + 1)
            if success:
                instance.cleaned = True
            instance.save()

the destroy_disks is false in ceph because ceph is shared block storage, and migrate_date is none in destroy_evacuated_instances scene. So the delete_instance_files method will not be executed.