Add optional ionice to volume clearing process

Allow the volume clearing process to have an ionice priority set to
reduce the performance impact of volume zeroing. Note that this may
cause volume clearing processes to get backed up on  a busy system, use
with care.

Change-Id: I2d556f57aaca8a8ccc6f0f767f1cec28c3f9bc86
Implements: when-deleting-volume-dd-performance