ibm_storage driver: the "OSvol:" prefix should be optional in the volume name

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The volumes created on DS8K by the ibm_storage driver all have a prefix "OSvol:" in the name.
And the DS8K storage has a limitation of maximum length of 16 for the volume name:
https://www.ibm.com/support/knowledgecenter/ST5GLJ_8.4.0/com.ibm.storage.ssic.help.doc/f2c_climkfbvol_1kywzf.html
https://www.ibm.com/support/knowledgecenter/ST5GLJ_8.4.0/com.ibm.storage.DS8000.msg/msg_CMUG00269E.html

The current behavior in ibm_storage driver is to add a "OSVol:" prefix and truncate the name if the combined name length is greater than 16.
/usr/lib/python3.6/site-packages/cinder/volume/drivers/ibm/ibm_storage/ds8k_proxy.py
```
            self.ds_name = (
                "OS%s:%s" % ('vol', helper.filter_alnum(self.cinder_name))
            )[:16]
```

In some case, we have common template for the volume name and the truncate will make the volumes created in DS8K backend all with the same name except the last few bits.

The "OSvol:" name prefix makes the customizable bits available less under the limitation of maximum length of 16.

Can we make this prefix as optional?

[Tushar Trambak Gite (tushargite96)]

prefix are already removed on this code
https://github.com/openstack/cinder/blob/1a231d3d2e9fcd3945b67cf7b8d7a8dda3c76e2a/cinder/volume/drivers/ibm/ibm_storage/ds8k_proxy.py#L233