Handle mutliline oslofmt traceback in logstash.

* modules/openstack_project/templates/logstash/indexer.conf.erb: Add a
mutliline filter that matches oslo format TRACE level log lines. This
will collect all lines related to the traceback in a single event.

Change-Id: I201e407b6bd6fbb5b2db94fa6757f184709f2eaa
Reviewed-on: https://review.openstack.org/32962
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins