Add Zaza target that discards Juju model after use

We do not want to keep the Juju models by default.

Change-Id: Ibe8b02002f1b88a222d958264b010e01e1ef3a17
diff --git a/tox.ini b/tox.ini
index 9b450ad..48c8a85 100644
--- a/tox.ini
+++ b/tox.ini
@@ -184,6 +184,12 @@
 commands =
     functest-run-suite --keep-model --bundle {posargs}
 
+[testenv:func-target-discard-model]
+basepython = python3
+passenv = {[defs]func_env_vars}
+commands =
+    functest-run-suite --bundle {posargs}
+
 [testenv:black-reformat]
 deps =
   black >= 22, < 23