qa/rbd/iscsi: ignore MON_DOWN warning in logs

These seem to be popping up very briefly and don't
appear to be relevant to what is being tested

Signed-off-by: Adam King <[email protected]>

Author: adk3798

qa/suites/rbd/iscsi/cluster/fixed-3.yaml

@@ -17,3 +17,7 @@ roles:
   - osd.7
   - client.2
   - ceph.iscsi.iscsi.b
+overrides:
+  ceph:
+    log-ignorelist:
+      - MON_DOWN