reuse existing task definitions

[wlggraham]

Old Behavior: Create a new task definition revision every time the plugin is called.

Problem: Even though this is the standard for ECS services, this quickly became an issue with standalone tasks, since the overriding resources (CPU & Memory) were constantly changing the task definition.

New Behavior: Allow new fields in job and command context:

• task_family: Override the existing name of the task family in your task definition if desired
• use_latest_revision: Instead of registering a new task definition, use the latest of the configured task_family
• task_revision: Use a specific revision

This will allow a few different desired outcomes:

• Have a single task definition template that can represent a handful of desired configurations without needing to register new task definitions every time.
• Pick a very specific revision if a rollback is needed for some reason