Better multiline handling for Jenkins console logs

* modules/openstack_project/templates/logstash/agent.conf.erb: Treat any
line not beginning with a timestamp in Jenkins console logs as belonging
to the previous log message. This keeps timestamps sane for messages
that do not have timestamps.

Change-Id: I5959e766c28efebbd6c44567262beb63ad3a0a8e
Reviewed-on: https://review.openstack.org/26777
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins