When we added concurrent disk erasures, we kept the concurrency
to 1 as to not risk any different oeprator behavior, at the cost
of not faster erasure times.
That being said, we have had the setting in place for some time
and we have received no reports of issues, so we are incrementing
it to four as that should be still quite relatively safe from a
concurrency standpoint for disk controllers in systems.
Change-Id: I6326422d60ec024a739ca596f46552bbd91b0419
556d5de9d3