Add support for attaching additional private networks to LBs

Factored out from #40.

Checklist

[x] Keep pull requests small so they can be easily reviewed.
[x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
[x] Link this PR to related issues.
[x] Rebase after #42 is merged