fix: add observation for sdcore management relation joined

[gatici]

Description

1.4/edge branch uses latest revision of nms image.
While running E2E sdcore-tests sometimes NMS stuck at installing agent status and sdcore-management-relation-changed hook fails.The reason of this failure is charm does not include storage paths of UPF and GNB config. As they are moved to webui which provides endpoints to configure UPF and GNB.
These are the related changes which causes the issue.

• feat: add webui backend to rock sdcore-nms#621
• feat: config gNBs and UPFs omec-project/webconsole#194

As a solution UPF and GNB config paths are removed from the charm.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[gatici]

This change does not solve the problem as NMS can not start independently using current image https://github.com/canonical/sdcore-nms/blob/main/rockcraft.yaml. The image is designed to run only Webui service. It is not easy to backport NMS- Webui combination to branch v1.4.