charts/nginz: Support galeb and allow upstream services to be in another namespace

[akshaymankar]

This is required to run galeb in Kubernetes.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md

[jschaul]

Needs a comment or two and a changelog, but seems to work as expected already.