Unable to remove volume type until image cache has been cleared

I noticed while writing Tempest tests for bug #1803961 that Tempest wasn't able to reliably clean up volume types while volumes using the type were still present in the image volume cache:

Introduce an attached volume migration test
https://review.openstack.org/#/c/637527/

http://logs.openstack.org/27/637527/12/check/tempest-slow/25e335a/job-output.txt.gz#_2019-04-04_14_18_07_474786

http://logs.openstack.org/27/637527/12/check/tempest-slow/25e335a/controller/logs/screen-c-api.txt.gz?#_Apr_04_14_18_07_105846

Reviewing the cinder code in this area shows that these cache volumes need to be manually deleted by an admin in order for them to then be considered evicted from the cache and no longer in use:

https://github.com/openstack/cinder/blob/6912b5f246bf3a12cab77d323a64d13e40d99ab4/cinder/volume/api.py#L502-L505

Shouldn't we automatically remove these when the last volume associated to this cached volume is removed?

Running the following Tempest scenario test locally shows this behaviour even while the overall test passes (I'm assuming Tempest is swallowing the failure somewhere?):

$ cd tempest
$ tempest run --regex tempest.scenario.test_volume_migrate_attached.TestVolumeMigrateRetypeAttached.test_volume_migrate_attached
[..]
{0} tempest.scenario.test_volume_migrate_attached.TestVolumeMigrateRetypeAttached.test_volume_migrate_attached [43.161687s] ... ok

$ openstack volume list --all
+--------------------------------------+--------------------------------------------+-----------+------+-------------+
| ID | Name | Status | Size | Attached to |
+--------------------------------------+--------------------------------------------+-----------+------+-------------+
| 2c48d377-4a3c-462d-9791-62df3635073f | image-4718064c-fa76-4ffa-80c1-44c8f1eef030 | available | 1 | |
+--------------------------------------+--------------------------------------------+-----------+------+-------------+

$ openstack volume type list
+--------------------------------------+----------------------------------------------------------------+-----------+
| ID | Name | Is Public |
+--------------------------------------+----------------------------------------------------------------+-----------+
| 92642ce1-0bb3-46e5-a731-4f8593d21e63 | tempest-volume-type-TestVolumeMigrateRetypeAttached-1047281271 | True |
| 60827973-eae8-4c2d-9774-5013ad4b05c4 | lvmdriver-2 | True |
| 6809c94a-ac65-481e-ab2a-01b7fc2fbc00 | lvmdriver-1 | True |
+--------------------------------------+----------------------------------------------------------------+-----------+

$ openstack volume type delete tempest-volume-type-TestVolumeMigrateRetypeAttached-1047281271
Failed to delete volume type with name or ID 'tempest-volume-type-TestVolumeMigrateRetypeAttached-1047281271': Target volume type is still in use. (HTTP 400) (Request-ID: req-83ce605a-f9fc-4e66-924f-32fb8183d4d3)
1 of 1 volume types failed to delete.

If this is expected then we can mark this as a bug against Tempest as it would need to remove any remaining volumes from the image volume cache before attempting to delete the volume type.