Set default value for tempest log file
Set default value for tempest log file

This commit changes the default value for tempest's logging output.
Since tempest is intended to be run solely through a test runner
logging to stdout can get complicated since tempest doesn't really
own stdout. This patch changes the default to log to tempest.log in
the cwd where tempest is run which should be a better behavior for
running tempest.

Change-Id: I1017a1714181a78b5da9c47e811db63b4edce14a

sebrandon1/tempest,Juniper/tempest,openstack/tempest,zsoltdudas/lis-tempest,masayukig/tempest,sebrandon1/tempest,cisco-openstack/tempest,bigswitch/tempest,openstack/tempest,LIS/lis-tempest,bigswitch/tempest,zsoltdudas/lis-tempest,cisco-openstack/tempest,vedujoshi/tempest,masayukig/tempest,vedujoshi/tempest,Tesora/tesora-tempest,LIS/lis-tempest,Tesora/tesora-tempest,Juniper/tempest