Support firelens for bridge mode ServiceConnect task

[yinyic]

Summary

Currently, if a task is ServiceConnect-enabled, and runs in bridge network mode, then it won't be able to use awsfirelens log driver.
This PR makes awsfirelens compatible with SC-enabled, bridge mode tasks.

Implementation details

ECS Agent is responsible for injecting FLUENT_HOST environment variable to application containers. The variable contains the IP address assigned to the FireLens container. See public doc. So far, ECS Agent resolves the container IP by directly querying the container network info from docker inspect. This becomes an issue with ServiceConnect bridge mode tasks, because we are launching pause containers for each application container (for network namespace configuration). In that case, the application container will not have the expected network info, as the info is now associated with its pause container.

This PR makes changes in two places -

1. When resolving container IP for FireLens container itself
2. When resolving FireLens container IP for other application containers

In both cases, we now retrieve the container IP from the FireLens pause container instead, using the readily available GetBridgeModePauseContainerForTaskContainer function.

Testing

• Added unit test for CreateContainer and StartContainer for both SC and non-SC bridge mode tasks.

• E2E test by running a bridge task with SC enabled that also use awsfirelens log driver - verified that 1) task is able to run, and 2) application log was successfully delivered to my fluentd destination (an S3 bucket)

Description for the changelog

Support firelens for bridge mode ServiceConnect task

Licensing

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