Remove --async option from puavo-update-client, it is not used
anywhere and is not very useful.
Allow puavo-update-client to fail in corresponding upstart service.
After an hour it will be tried again.
Make puavo-update-client use the new dbus-interfaces as they are
intended.
Return some string in Update-dbus-method.  Perhaps these messages
are useless and could be removed from the interface?