Remove deprecated scenario image option

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: I949cbc6347030461a2daf0658d1ffef1d7aa0e87

releasenotes/notes/Remove-deprecated-image-scenario-option-c67490ab7f9d4f92.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    Remove ``CONF.scenario.img_dir`` option as it is being removed from
+    Tempest after ~4 year deprecation period.
+    ``CONF.scenario.img_file`` option needs to contain the full path to an
+    image to upload to glance.

templates/tempest.conf.j2

@@ -128,8 +128,7 @@ lock_path = {{ tempest_venv_bin | dirname }}/locks
 
 
 [scenario]
-img_dir = {{ tempest_image_dir }}
-img_file = {{ tempest_images[0]['url'] | basename }}
+img_file = {{ tempest_image_dir }}/{{ tempest_images[0]['url'] | basename }}
 img_container_format = bare
 img_disk_format = {{ tempest_images[0]['format'] }}