Document how to make tests log at DEBUG level
Document how to make tests log at DEBUG level

OS_DEBUG is a handy trick, so let's make it slightly more
discoverable.

Change-Id: I02e4fbffc3c2c4f5b2737a44581ba7168f3a0d0f

klmitch/nova,openstack/nova,rahulunair/nova,rahulunair/nova,klmitch/nova,mahak/nova,rahulunair/nova,mahak/nova,klmitch/nova,klmitch/nova,openstack/nova,openstack/nova,mahak/nova