Add logstash documentation.

* doc/source/logstash.rst: Add documentation on our Logstash system
architecture and how to query logstash.

Change-Id: I9da3e6d6391081131d1fd852230ddac6326c01a2
Reviewed-on: https://review.openstack.org/31257
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins