Make ingress-per-app compatible with ops 2.10.0

[natalian98]

Enhancement Proposal

Ingress-per-unit relies on ops 2.10 behaviour that breaking relations don't show up in model.relations.
A similar fix is needed for ingress-per-app, see this conversation for reference.

[PietroPasotti]

By looking at the traceback, I don't see how ingress-per-app is related.
I think the issue you're having is that adding a relation to the harness doesn't automatically add a network for it, which the library (rightfully) expects.
Are you saying this changed with some recent release of ingress, or of ops?

Maybe it's behaviour that has changed in ops 2.10 with respect to 2.9, but I don't recall seeing anything about this.

[PietroPasotti]

Fixed by: canonical/operator#1138