Fix : get_retention_data

* 'prop' variable should not be enclosed by quote
* The perf data property for a host is process_perf_data not process_performance_data
Fix : Revert DICT_MODATTR modificiation and ensure property is defined in DICT_MODATTR before use
Switched placeholder file to .gitempty
Removed .gitempty from configuration directories
Renamed to .gitkeep
fix: bad super call. would infinitely recurse if subclassed..
Merge pull request #1402 from gst/fix_bad_super

fix: bad super call. would infinitely recurse if subclassed..
Enh : Rename shinken script for BSD and remove useless README
Fix typo in Passive checks documentation page
Merge pull request #1417 from dfranco/patch-1

Enh : Doc - Fix typo in Passive checks documentation page
Fix #1416
Update for 2.2 and clean tab
Enh : Changelog - Clean tab
Merge pull request #1365 from AddOnline/fix-get-retention-data

Fix : get_retention_data function in order to allow retention module to work
Merge pull request #1397 from OpenConceptConsulting/feature/gitempty

Enh : Switched placeholder file to .gitempty