CI: make the ansible inventory a directory

Not constraining the inventory to .ini allows us to use dynamic
inventory, which is needed for simplifying kubevirt jobs inventory.

Also reduces the scope of the ANSIBLE_INVENTORY variable.

tests/scripts/testcases_cleanup.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -euxo pipefail
 
-cd tests && make delete-${CI_PLATFORM} -s ; cd -
+make -C tests delete-${CI_PLATFORM} -s
 
 if [ -d ~/.ara ] ; then
   tar czvf ${CI_PROJECT_DIR}/cluster-dump/ara.tgz ~/.ara