Do not hard code builder image locations

[lance]

The buildpack Runtimes are all currently hard coded.

https://github.com/boson-project/faas/blob/566d8f9255d532e88e72d5bce122bebaee88bc81/buildpacks/builder.go#L25-L29

It would be nice for this to be configurable.

[lance]

As I think more about this issue, it's making me realize that all of the actions this CLI takes are really not function-specific. Making the runtime buildpacks configurable would mean that this CLI could extend the same build/deploy/update/delete workflow to essentially anything that could be deployed as a Knative service and which has a buildpack. Want to run a Django app on Knative? Point faas at https://github.com/cloudfoundry/python-buildpack and it's easy.

@boson-project/core wdyt?

[lance]

What if, instead of configuring builders explicitly, we simply add metadata to the templates. For example:

❯ cat templates/node/http/.builder     147ms  Fri 18 Sep 2020 03:49:45 PM EDT
───────┬──────────────────────────────────────────────────────────────────────
       │ File: templates/node/http/.builder
───────┼──────────────────────────────────────────────────────────────────────
   1   │ quay.io/boson/faas-nodejs-builder
───────┴──────────────────────────────────────────────────────────────────────