charm/zaza: check pipeline, let Zuul do its job

Move the charm test job to Zuul's "check" pipeline so that it can react
automatically on new patchsets and Code Review / Workflow votes.

Change-Id: Ie9cda167b944b6c2e15efe08daf856676c8aba31
diff --git a/zuul.d/charm-cinder-storpool-iscsi.yaml b/zuul.d/charm-cinder-storpool-iscsi.yaml
index 21b5e95..eff69fb 100644
--- a/zuul.d/charm-cinder-storpool-iscsi.yaml
+++ b/zuul.d/charm-cinder-storpool-iscsi.yaml
@@ -86,7 +86,7 @@
 
 - project:
     name: charm-cinder-storpool-iscsi
-    experimental:
+    check:
       jobs:
         - zaza-functional-focal-ussuri:
             branches: master