changelog: add private registry auth over asm
# Changelog
+## 1.19.0-dev
+* Feature - Private registry can be authenticated through task definition using AWS Secrets Manager [#1427](https://github.com/aws/amazon-ecs-agent/pull/1427)
+
## 1.18.0
* Feature - Configurable container image pull behavior [#1348](https://github.com/aws/amazon-ecs-agent/pull/1348)
* Bug - Fixed a bug where Docker Version() API never returns by adding a timeout [#1363](https://github.com/aws/amazon-ecs-agent/pull/1363)
