Register user-defined resources per container instance

The ECS API allows container instances to track user-defined resources beyond cpu and memory. From http://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_Resource.html:

> name: The name of the resource, such as CPU, MEMORY, PORTS, or a user-defined resource.

The ecs-agent's calls to RegisterContainerInstance should provide some mechanism to register user-defined resources. [The API call the agent makes allows this](http://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_RegisterContainerInstance.html), but as far as I know there's no way for a user to control this aspect of the registration process.

Coupled with TaskDefinition support for reserving user-defined resource types, this would allow users to have much more control over task scheduling in a custom system via [StartTask API calls](http://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_StartTask.html).

My specific interest is in allowing my tasks to reserve disk space on their hosts.

Start by reviewing the ECS RegisterContainerInstance, TaskDefinition, and StartTask API entry points referenced in the issue, focusing on how user-defined resources would be registered and reserved. Done would be a defined, usable mechanism for registering custom resources such as host disk space and reserving them for tasks.

Written by the indexing model from the issue text.