Allow (or force) upload of all images to user's private ECR during cluster up

[deliahu]

Description

Part of the cortex cluster up process would be to upload the appropriate version of all Cortex images to the user's private ECR.

It might be worth considering this in conjunction with #633

Questions:

• What to name the ECR repo prefix?

  • cortex/python-predictor-cpu
  • cortex-cluster/python-predictor-cpu

• Should there be a placeholder that users can use to reference the ECR repo in their region?

  • $my-ecr/cortex/python-predictor-gpu-slim:0.22.1-cuda10.1-cudnn8
  • $my-ecr-repo/cortex/python-predictor-gpu-slim:0.22.1-cuda10.1-cudnn8

Motivation

Remove external dependencies after the cluster has been created, reduce image pull time