Allow predictor base images to be overridden in API configuration

[deliahu]

Motivation

• Different APIs may benefit from having different base images
• Allow rolling updates when updating the base image

Design

Add a top-level key under predictor image

- name: iris-classifier
   predictor:
    type: python
    path: predictor.py
    image: ecr/dockerhub link

• image defaults to cortex default images if user doesn't specify key
• validate that the image path, check access to image (prioritize DockerHub, we may be able to add ECR later)

[RobertLucian]

Per our last conversation, I'm grabbing this ticket. Thanks @vishalbollu !

Useful articles:

• http://blog.shippable.com/setting-permissions-on-aws-ec2-ecr-repositories