Allow cli overrides of render params

[grantkellyADSK]

Summary

Allow command line overrides of render params

Motivation

Overriding values is a common requirement when building reusable pipelines. Allowing values in an existing render.yml file to be overridden at the command line gives flexibility without having to rewrite the yaml file programmatically. jsonnet supports both "external variables" and "top-level arguments" as command line arguments.

Describe alternatives you've considered

1. Rewriting the render.yml file before running shore render

Additional context

An example of the proposed syntax:

shore render --override foo=bar

where foo is a YAML key path, and bar is a YAML value.