Set the scenario/img_dir option for tempest.conf

The default value does not work unless devstack is installed in
/opt/stack/new/devstack. This patch changes the img_dir option to point
to the correct install location.

Change-Id: Iea41d209dad10b2f9a7c97efd55c39a8d29347cc
Merge "Set the scenario/img_dir option for tempest.conf"