Configuration option documentation update

[Nick Barcet]

As we switched to multi-publisher, some of the old configuration options such as "periodic_interval" were made obsolete, but not remove [1] not was the pipelining explained nor its configuration documented. We would also need a pointer to [2] as we did for [3] at the bottom of [1].

[1] http://docs.openstack.org/developer/ceilometer/configuration.html
[2] https://github.com/openstack/ceilometer/blob/master/etc/ceilometer/pipeline.yaml
[3] https://github.com/openstack/ceilometer/blob/master/etc/ceilometer/ceilometer.conf.sample

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/33823
Committed: http://github.com/openstack/ceilometer/commit/3ed819cb2bb0760bb4a9f2d0c03451e1afa8cd99
Submitter: Jenkins
Branch: master

commit 3ed819cb2bb0760bb4a9f2d0c03451e1afa8cd99
Author: Julien Danjou <email address hidden>
Date: Thu Jun 20 15:53:08 2013 +0200

    Remove useless periodic_interval option

    Change-Id: Iac2c72bf64be79d1227af5b98873b5b1511b548a
    Fixes: bug#1181160

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/grizzly
Review: https://review.openstack.org/39479