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
Merge pull request #1365 from AddOnline/fix-get-retention-data

Fix : get_retention_data function in order to allow retention module to work