Set dynamicTriggerConfiguration switch

In Jenkins version 1.490 dynamicTriggerConfiguration
needs to be explicitly turned off in order for the
trigger-on-patchset-uploaded-event flag to work
properly.

Change-Id: If5835e933b812a7fe17c49f0f6988360f7154f80
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Do not assume a failure message is specified

Not everyone needs a custom failure message, so
if nothing is specified create an empty node.

Change-Id: I82f69e28574c01cbc6de649f968482352f42b8d8
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>