Update docs to highlight that the auditor chunk size can be set

May not be obvious, but existing code will let you change the
disk_chunk_size just for the auditor so this just points that
out in the docs.  In one short test I ran with a 4 node cluster
with 18GB of 4MB objects on it, changint he auditor chunk size
from the default of 64K to 1MB creased the auditor CPU time from
10% to 4%.

Also added test code to make sure this overridden value is
actually used and checked other auditWorker conf values as
well.

Change-Id: Ia12e1c6127877dc2124b60cd963cd0b6d5f3d6ef