Enable debug logging on tempest

With tempest moving to testr (serially currently) the log level is no
longer defaulting to debug as it did with nose. To get the same level
of verbosity in the logging as when running with nose this commit
sets the debug flag on tempest.

Change-Id: I6acd57be0f8188d31825d88471ba9883ebb30519
Merge "Enable debug logging on tempest"
Merge branch 'master' of github.com:openstack-dev/devstack