Remove tempest deprecated img_dir option

Tempest option scenario.img_dir has been deprecated more than 4 years, it's
time to remove it from devstack.
img_file option should contain the full path to the image.
This patch removes setting of img_dir option and makes img_file
one contain the full path of an image.

Change-Id: I71102095f3603915f0bc7d21f2e18c4eac4e95ec
Depends-On: https://review.opendev.org/#/c/710996/
Related-Bug: #1393881
diff --git a/lib/tempest b/lib/tempest
index 05fcb1f..0d6bdc9 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -433,8 +433,7 @@
         SCENARIO_IMAGE_DIR=${SCENARIO_IMAGE_DIR:-$FILES}
         SCENARIO_IMAGE_FILE=$DEFAULT_IMAGE_FILE_NAME
     fi
-    iniset $TEMPEST_CONFIG scenario img_dir $SCENARIO_IMAGE_DIR
-    iniset $TEMPEST_CONFIG scenario img_file $SCENARIO_IMAGE_FILE
+    iniset $TEMPEST_CONFIG scenario img_file $SCENARIO_IMAGE_DIR/$SCENARIO_IMAGE_FILE
 
     # If using provider networking, use the physical network for validation rather than private
     TEMPEST_SSH_NETWORK_NAME=$PRIVATE_NETWORK_NAME