Add missing argument in notify_callback

This fix is related to bug 866 and caused an exception when
the charm was not found. Nonetheless it did not fix the bug.
https://osm.etsi.org/bugzilla/show_bug.cgi?id=866



Change-Id: I641834461ac508eac131c32a4c4ef3557e192b1a
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>