Change CSV file date format to Horizon standard

This patch alters the date format to be 'YYYY-mm-dd', in line with the Overview
query directly by the "Download CSV Summary" button, and the rest of
Horizon.

Change-Id: I39b3bd68eed48ee3b4aace6e75d11f56b69decf4
Closes-Bug: 1387744
Merge "Change CSV file date format to Horizon standard"