fix: go into blocked status if MetalLB is not enabled and N2 relation is set

[dariofaccin]

Description

This PR aims to fix #45. The fix catches the TypeError exception thrown when trying to access ingress information when MetalLB is not enabled or properly configured, and leverages the validation from the N2 interface to set the charm into blocked status.

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]

Could we consider to add an integration test which sets a relation with gnbsim ?

[dariofaccin]

Could we consider to add an integration test which sets a relation with gnbsim ?

At the moment integration tests involves only mandatory relation: actually, we have the AMF-nGBsim integration test in the sdcore-gnbsim-operator repo.
It also is not related to the scope of this PR: please open a feature request for that.

[Gmerold]

Looks like I'm too late, but I don't we should be mentioning MetalLB specifically, because that's gonna be only valid for MicroK8s. What about other LB services?