Add changelog for agent 1.28.0.
# Changelog
+## 1.28.0-dev
+* Feature - Introduce high density awsvpc tasks support
+* Enhancement - Introduce `ECS_CGROUP_CPU_PERIOD` to make cgroup cpu period configurable [@boynux](https://github.com/boynux) [#1941](https://github.com/aws/amazon-ecs-agent/pull/1941)
+* Enhancement - Add Private Host IPv4 address to container metadata [@bencord0](https://github.com/bencord0) [#2000](https://github.com/aws/amazon-ecs-agent/pull/2000)
+* Enhancement - Set terminal reason for volume task resource [#2004](https://github.com/aws/amazon-ecs-agent/pull/2004)
+* Bug - Fixed a bug where container health status is not updated when container status isn't changed [#1972](https://github.com/aws/amazon-ecs-agent/pull/1972)
+* Bug - Fixed a bug where containers in 'dead' or 'created' status are not cleaned up by the agent [#2015](https://github.com/aws/amazon-ecs-agent/pull/2015)
+
## 1.27.0
* Feature - Add secret support for log drivers
