Add dependency to ECS agent service

[lewayne-aws]

Issue #, if available:
n/a

Description of changes:
If ECS starts before credentials-fetcher, the agent fails to connect to the credentials-fetcher socket. So, when installing credentials-fetcher.service, add a drop-in configuration for ecs.service, so that credentials-fetcher.service effectively becomes a component of ecs.service.

With this change, if ecs.service is started, stopped, or restarted, credentials-fetcher.service will also do the same, and ecs.service will wait for the socket file to be created when starting credentials-fetcher.service.

Since ecs.service becomes so tightly coupled with credentials-fetcher.service, we don't have to explicitly enable it for it to start on systems where the ECS agent is installed. Instead, ecs.service triggers the service to start with it as a dependency.

If this is installed on a system where ECS agent is not installed, there are no side effects. Although there would be a drop-in file for ecs.service, without the underlying unit file also present (provided by ECS agent), the drop-in will be ignored by systemd. Similarly, this approach does not clobber any of the files provided by ECS agent directly, and simply leverages existing systemd functionality to enforce our intent and requirement.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[smhmhmd]

Can you add to the changelog as well

[heathhey]

After a review of the pull request, I think if you want to use systemd to enforce ordering of startup and restart etc. then this is the best solution. I have not personally setup or used this solution, so I recommend testing this setup. with this configuration, if the credentials-fetcher service dies, or is stopped or is restarted, then the ecs service will follow suit, this is what the BindsTo option does.

I would highly recommend testing this thoroughly. Doing something like, making sure both packages are installed, and that the service is running, then force a restart or a stop and stare on credentials-fetcher and verifying that the ecs service follows along. Also, a test with a custom AMI with both installed to verify that it does solve the race condition and the both services start properly.