DataSourceOVF: do not log exception on failed mount

during datasourceovf looking for a transport, the failed
mounts were having exceptions logged (triggering an unneccessary
warning also)

cloudinit/sources/DataSourceOVF.py

@@ -213,8 +213,7 @@ def transport_iso9660(require_iso=True):
             (fname, contents) = util.mount_cb(fullp,
                                                get_ovf_env, mtype="iso9660")
         except util.MountFailedError:
-            util.logexc(LOG, "Failed mounting %s", fullp)
-            continue
+            log.debug("%s not mountable as iso9660" % fullp)
 
         if contents is not False:
             return (contents, fullp, fname)