fix config parsing in swift-bench -x
fix config parsing in swift-bench -x

This patch ensures that the command-line arg format (boolean)
doesn't conflict with the conf file format (string) and the
proper action is taken.

Change-Id: I3284096e1a9478897e1c3246ab190b46d2590243

jeblair/swift-bench,tobegit3hub/swift-bench,openstack/swift-bench,openstack/swift-bench,tobegit3hub/swift-bench