StorPool: encrypted volumes cannot be attached

The StorPool Cinder driver still overrides the internal _attach_volume() and _detach_volume() methods, even after the base driver was modified to use os-brick for attaching volumes locally e.g. for transferring Glance image data. This, coupled with the fact that _attach_volume() (through _connect_device()) is now supposed to also return connection information, leads to an attempt to access missing connection information after attaching an encrypted volume.

The fix is trivial: remove the _attach_volume() and _detach_volume() methods from the StorPool driver, as done in https://review.opendev.org/c/openstack/cinder/+/787296

The patch from the above review has been in use in the StorPool driver's CI system since April; actually, the Cinder/Nova encrypted volume tests did not pass without it. We believe that the "cannot attach encrypted volumes" bug (due to the missing "conn" member of the returned structure) has been present at least since the Newton release, so the fix should probably be backported at least to Wallaby, if not earlier.

Thanks in advance for your time, and keep up the great work!