container associations handler and task server setup

[cyastella]

Summary

• This add the outline for the container associations handlers in v4 which align with v3 endpoint.
• Create v4 endpoint for the container with associations and set up task server.

Implementation details

• In the association handler file we specify the container association(s) path that users used to query container association(s) for v4.

• For the container associations info, these only apply to the awsvpc task with elastic-inference set up.

• Customers curl the paths like:
  v4/endpoint-id/associations/elastic-inference
  v4/endpoint-id/associations/elastic-inference/(device-id)

• Example response:

curl v4/endpoint-id/associations/elastic-inference

{"Associations":["device_1"]}

curl v4/endpoint-id/associations/elastic-inference/(device-id)

{
	"version_2018_04_12": {
		"elasticInferenceAcceleratorId": "eia-8aa24aa24f05423c8bf647de750c24b3",
		"elasticInferenceAcceleratorType": "eia1.large",
		"deviceId": "device_1"
	}
}

Testing

New tests cover the changes: yes

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.