Make pullImageTimeout configurable

[ubhattacharjya]

Summary

This code change makes the image pull timeout configurable. The default timeout if not mentioned is 2h for both linux and windows. However by mentioning the environment variable ECS_IMAGE_PULL_TIMEOUT in ecs.config file you can configure the timeout according to your wish

Implementation details

Testing

Unit tests cover changes
Tested the following manually on AL2, AL1 and windows instance:

1. If not mentioned, default timeout is getting considered.
2. If mentioned, the new timeout is getting considered.
3. If mentioned to a really small value, the docker image pull process is correctly timing out and the task fails

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.