Set a message_format for logstash log-pusher input

* modules/openstack_project/templates/logstash/agent.conf.erb: Set
message_format to "%{@message}".

Change-Id: Ia8e753ff8bc9a2e0b16a6b43a7cbaf74c6a59eb2
Reviewed-on: https://review.openstack.org/26412
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: K Jonathan Harker <jesusaurus@mail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins