Increase Jenkins slave syslog max message size.

To better capture python tracebacks with syslog set the rsyslog
$MaxMessageSize to 64k. This change is being made to better accomodate
devstack-gate logging after we have stopped archiving the devstack
screen logs.

Change-Id: Ibb01f759f111d924a21c20f8521ea5db4a35e676
Reviewed-on: https://review.openstack.org/21233
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins