Add GCR helper script to set up credentials for local namespace

[evankanderson]

Fixes #21

This automates the creation of GCP credentials, permissions application on GCR buckets, and creation of kubernetes secrets in the current namespace using the _json_key style of secret.

Sorry about the shell script -- it seemed like the most readable/useful option, though the jq usage is a bit tricky.

[enisoc]

Could we have a way to specify which k8s namespace to put things in? I'd rather not have to change the default namespace for my kubectl context just for this step.

[adrcunha]

Why not putting this script in //hack, together with the other scripts?

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[evankanderson]

Re @enisoc -- added positional arguments for namespace ($1) and kubernetes SA ($2) Re @adrcunha -- this script is intended for use by operators and/or developers who are looking to use the builds, not for contributors, which is what I'd expect //hack to be for. Additionally, build-templates doesn't have a //hack directory at all.

Updated, PTAL.

[evankanderson]

Updated docs, do you want to take a look again, @ImJasonH

[adrcunha]

/lgtm /woof space