Add the "<filter></filter>" only when logfile environment Variable is set

[ubhattacharjya]

Summary

This PR fixes the empty (filter)(/filter) when ECS_LOGFILE is not set by ecs-init. This would cause certain environment variables related to agent logging to not have their expected behaviour when the user is not using ECS optimized AMI.
This issue is mentioned in #2591

Problem:

For example,

$ docker run -e ECS_LOG_OUTPUT_FORMAT=json amazon/amazon-ecs-agent:v1.44.0
1598513604929243900 [Error] node must have children

This error is caused because ecs agent is receiving empty (filter)(/filter) when ECS_LOGFILE is not set by ecs-init.

However, when the below is run, everything is fine.

docker run -e ECS_LOG_OUTPUT_FORMAT=json --env=ECS_LOGFILE=/log/ecs-agent.log amazon/amazon-ecs-agent:v1.44.0
{"level": "info", "time": "2020-08-28T21:08:40Z", "msg": "Loading configuration", "module": "agent.go"}

Implementation details

This PR adds the filter block only when ECS_LOGFILE is present as env var. Otherwise it is not added

Testing

Unit tests cover the changes
Manual test:

[ec2-user@ip-172-31-3-170 ~]$ docker run -e ECS_LOG_OUTPUT_FORMAT=json amazon/amazon-ecs-agent:latest
{"level": "info", "time": "2020-08-28T22:24:32Z", "msg": "Loading configuration", "module": "agent.go"}

Also tested that the /var/log/ecs/ecs-agent.log file also got populated in json format when ECS_LOG_OUTPUT_FORMAT=json is mentioned ecs.config file

Description for the changelog

Licensing

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

[mssrivas]

Can we add a test/unit test to make sure this works as we think it should?

[ubhattacharjya]

Can we add a test/unit test to make sure this works as we think it should?

added unit tests for testing a scenario when logfile is not mentioned