Change FAAS_REPOSITORY to FAAS_REGISTRY

[lance]

I think probably it makes sense as @rhuss suggested, to change FAAS_REPOSITORY to FAAS_REGISTRY since that's really what we are asking for anyway (terminology in this space is so messy).

A simple Quarkus project faas.yaml looks like this:

name: quarkus
namespace: ""
runtime: quarkus
image: docker.io/lanceball/quarkus:latest
trigger: events
builder: default
builderMap:
  default: quay.io/boson/faas-quarkus-jvm-builder
  jvm: quay.io/boson/faas-quarkus-jvm-builder
  native: quay.io/boson/faas-quarkus-native-builder

Instead, I think the image property should just be quarkus:latest and that we should persist the FAAS_REGISTRY value in this file as well.

/cc @boson-project/core

[lance]

Alternatively, we could get rid of FAAS_IMAGE and instead keep FAAS_REPOSITORY but have it point to the full image, meaning registry/[user]/<repository>:[tag] (where [user] and [tag] are optional (since a registry may not expose repos in a namespace and tag could just be latest).