collectd-nagios: If DSs are specified, sort the values accordingly.

collectd-nagios: If DSs are specified, sort the values accordingly.

If a user specifies DSs on the command line, the order or the values might
matter (e.g. when using the upcoming "percentage" consolidation function). So,
in this case we now order the values according to the order of the specified
DSs.

This patch replaces the function ignore_ds() with filter_ds() which is called
once in do_check(). This removes the need to check for ignored values in all
consolidation function implementations.