Logging configuration

Add some documentation on logging configuration.

This content is based on a blog entry by Sébastien Han:
http://www.sebastien-han.fr/blog/2012/12/05/openstack-and-rsyslog/

Backported from https://review.openstack.org/19047

Change-Id: Ic8f2007699681a0c17184fa538e3133dec4edf86