[VNX] Unable to delete volume because of StorageGroup

[TommyLike]

We had this issue on Mitaka version, feel free to add fix link and close it if it's already been fixed in the newer version.

Environment: OpenStack Mitaka
Driver: EMC VNX
Issue: When detach and delete the volumes, the terminate_connection operation would succeed with warning:

```
No Such Host LUN in this Storage Group, see attachment (image_006)
```

but then when we try to delete the volume the command would fail with result:

```
Can't not unbind LUN, because it's contained in a Storage Group, see attachment (image_001)
```

[Yong Huang (yonghuang)]

Hi,

We can't root cause this issue by the Errors in the pictures.

Could you provide more details:
1. Is there full cinder logs? We need to check whether is the LUN attached correctly.
2. What is the test steps and scenarios?
3. What is the concurrency number of LUN attach, detach?