Removing deprecated CONF.scenario.img_dir option as it's beeing removed
from Tempest after ~4 year depreation period [1].
Currently Tempest is throwing a warning if img_dir is still used.
Both behaviors (img_dir and a full pathin img_file) are supported until
Tempest 25.0.0, starting Tempest 26.0.0 a full path in
CONF.scenario.img_file config option will have to be specified.
[1] https://review.opendev.org/#/c/710996/
Related-Bug: #1393881
Change-Id: Ifd02065de88eea1c75825be3178fc9f8d9721a16
4dbe3c2f69