don't forget to use actual command if needed...
more consistency in function names and goals

Keep logical volume info extraction in _getVolumeInfo(), renamed to
_getLogicalVolume() for consistency with similar functions, rename
original _getLogicalVolume() to _getLogicalVolumesFromGroup() and limit
it to returning a list of volume names.

see: #2384
test logical volume names extraction

see: #2384
cosmetics

see: #2384