Ensure cell config uses CellPortManager to resolve ports

What does this merge request do and why?

Prior this commit a cell config used the default PortManager which did not take the port offset into account when resolving service ports.

This commit sets cells.port_offset first before so CellsPortManager is used when resolving ports.

How to set up and validate locally

1. Enable cells and gdk cells up
2. rm -f gitlab-topology-service/config.toml - that's unfortunate but required :(
3. gdk reconfigure
4. Ensure that gitlab-topology-service/config.toml shows address = "<IP>:12001"

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.