Use "GetExport" method in the API

Warning: it is a breaking change in the API, so to be scheduled in Glances 5.0

For historical reason, the Glances Restfull API (and the XML one) do not use the new getAllExportsAsDict, get_export, getAllExport methods of the GlancesStats class. This new method filter the stats by taken into account the export_exclude_list variable of the Plugins.