Add log directory option to functional tests
Add log directory option to functional tests

This option to the functional test runner outputs logs for each test
to ${LOGDIR}/${element}/${test_element}.log

Change-Id: I3bf5362bcda8acda1a03c1058728875a9a6c9711

switch-ch/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder