Description of problem:
Allow the QOS volume retype of an in-use volume on a shutdown instance. While I can understand that the retype of an in-use volume on a running instance for QOS settings is forbidden, prohibiting this on a shut down instance is in my opinion a bug.

~~~
[stack@undercloud-1 ~]$ cinder show 4dddf8fa-8186-4b1b-bca3-0343e79c5f42 | grep server
| attachments                    | [{u'server_id': u'a17c2606-e701-4965-b8b0-2a270c201bbe', u'attachment_id': u'68ce8dc4-3449-4a2a-8e02-cd70dfe1ca3b', u'attached_at': u'2017-09-07T18:57:52.000000', u'host_name': None, u'volume_id': u'4dddf8fa-8186-4b1b-bca3-0343e79c5f42', u'device': u'/dev/vdb', u'id': u'4dddf8fa-8186-4b1b-bca3-0343e79c5f42'}] |
[stack@undercloud-1 ~]$ cinder retype 4dddf8fa-8186-4b1b-bca3-0343e79c5f42 iscsi-high
ERROR: Invalid volume: Retype needs volume to be in available or in-use state, not be part of an active migration or a consistency group, requested type has to be different that the one from the volume, and for in-use volumes front-end qos specs cannot change. (HTTP 400) (Request-ID: req-1f85f4a2-9768-45f7-a3f3-b4821d1505df)
[stack@undercloud-1 ~]$ nova stop a17c2606-e701-4965-b8b0-2a270c201bbe
Request to stop server a17c2606-e701-4965-b8b0-2a270c201bbe has been accepted.
[stack@undercloud-1 ~]$ sleep 15; nova list | grep a17c2606-e701-4965-b8b0-2a270c201bbe
| a17c2606-e701-4965-b8b0-2a270c201bbe | rhel-test1 | SHUTOFF | -          | Shutdown    | private=2000:192:168:1:f816:3eff:fe7b:7526, 192.168.0.7, 10.0.0.103 |
[stack@undercloud-1 ~]$ cinder list | grep 4dddf8fa-8186-4b1b-bca3-0343e79c5f42
| 4dddf8fa-8186-4b1b-bca3-0343e79c5f42 | in-use | test-volume2 | 5    | iscsi-low   | false    | a17c2606-e701-4965-b8b0-2a270c201bbe |
[stack@undercloud-1 ~]$ cinder retype 4dddf8fa-8186-4b1b-bca3-0343e79c5f42 iscsi-high
ERROR: Invalid volume: Retype needs volume to be in available or in-use state, not be part of an active migration or a consistency group, requested type has to be different that the one from the volume, and for in-use volumes front-end qos specs cannot change. (HTTP 400) (Request-ID: req-cb74629d-7355-47c7-a500-12b4297bdd91)
[stack@undercloud-1 ~]$ 
~~~