Add image format enforcement toggle

Related to blueprint glance-as-defender

Needed-By: https://review.opendev.org/c/openstack/tempest/+/931028
Change-Id: I8b22ed85eefde399f2e472780106dd39e51a5700
diff --git a/lib/tempest b/lib/tempest
index 310db2d..eeeef67 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -368,6 +368,7 @@
         if [[ -n "$image_conversion" ]]; then
             iniset $TEMPEST_CONFIG image-feature-enabled image_conversion True
         fi
+        iniset $TEMPEST_CONFIG image-feature-enabled image_format_enforcement $GLANCE_ENFORCE_IMAGE_FORMAT
     fi
 
     iniset $TEMPEST_CONFIG network project_network_cidr $FIXED_RANGE