fix(nginz): also include namespace in upstream name when upstream_namespace is set

[lwille]

If upstream_name is set, we're including it in the proxy_pass directive. However, the actual upstreams never included the namespace name.
This PR adds the namespace to the upstream name in the upstream declaration, so that proxy_pass is not running into a 504 error for such services.

related to WPB-19292

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines