add a logs directory help file

this adds more details on debugging at a log level, it only currently
has tips for nova and cinder on the hope that others will add in
tips for their services as well.

Change-Id: I056979344ba8903420b8b096298cd76f6a5c75ce
Reviewed-on: https://review.openstack.org/33691
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins