Use gearman to distribute logstash log pushing.

This change reorgs the logstash log pushing so that there is a central
gearman server that listens to Jenkins ZMQ events which are then
converted to per log file gearman jobs which are processed by gearman
workers. The central gearman server will live on logstash.o.o and the
existing logstash-worker hosts will be converted to gearman log pusher
workers.

This commit includes relavent documentation changes.

Change-Id: I45f7185c2479c54b090d223408dff268e1e8d7db
Reviewed-on: https://review.openstack.org/32455
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins