Updated harness so you can command-line override file configs.

Change-Id: I788a30f647d75681b65bf5e6dec40b1d4c20a3bc
Reviewed-on: http://review.couchbase.org/31756
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>