Fix image file path of upgrade-tempest

 Currently, boto's 's3_materials_path' and scenario's 'img_dir' of
tempest.conf values are incorrect because the global variable of FILES
is not set and not passed to the devstack's configure_tempest().
 This commit fixes it to be set the proper value.

Change-Id: Ic926d8776155a580fd84ad8d2d49b6007954de4f