Add ecs-agent container healthcheck localhost ip override env var

[fierlion]

Summary

This change allows IP configuration for the ecs-agent container's docker healthcheck localhost IP to address a specific case where the customer has overridden the localhost ip explicitly.

Implementation details

Currently, as the agent container is launched in host network mode, when the localhost inside the scratch-based agent container does not resolve to that the same IP, the agent container healthcheck will always return 'unhealthy'.
This change allows the localhost IP to be explicitly set to either the host localhost ip (usually 127.0.0.1) or otherwise based on DNS resolution requirements.

Testing

I built the agent and tested the following manually:
start agent the environment variable unset -- agent container became healthy
set ECS_AGENT_HEALTHCHECK_HOST=localhost -- agent container became healthy
set ECS_AGENT_HEALTHCHECK_HOST=127.0.0.1 -- agent container became healthy
set ECS_AGENT_HEALTHCHECK_HOST=80.80.80.80 -- agent container became unhealthy.

New tests cover the changes: no

Description for the changelog

Add configurable agent healthcheck localhost ip env var.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sparrc]

there are some existing unit tests for the healtcheck, does it make sense to add a test or two for this? https://github.com/aws/amazon-ecs-agent/blob/90ccb1d4313eb853691465197d324cd76404b1cc/agent/app/healthcheck_test.go