_json_key is never set to USERNAME (#950)
_json_key is never set to USERNAME (#950)

* Update docker-registry.md

`USERNAME` variable can't be overridden (in my Mac at least). So, `_json_key` is never used.

* Remove unused USERNAME assignment

spinnaker/spinnaker.github.io,spinnaker/spinnaker.github.io,ttomsu/spinnaker.github.io,spinnaker/spinnaker.github.io,ttomsu/spinnaker.github.io,ttomsu/spinnaker.github.io