Check/get ingress port on add-on load (#4744)

Instead of setting the ingress port on install, make sure to set
the port when the add-on gets loaded (on Supervisor startup and
before installation). This is necessary since the dynamic ingress
ports are not stored as part of the add-on data storage themself
but in the ingress data store. So on every Supervisor start the
port needs to be transferred to the add-on model.

Note that we still need to check the port on add-on update since
the add-on potentially added (dynamic) ingress on update. Same
applies to add-on restore (the restored version might use a dynamic
ingress port).

tests/fixtures/addons/local/example/config.yaml

@@ -18,3 +18,5 @@ options:
   message: "Hello world..."
 schema:
   message: "str?"
+ingress: true
+ingress_port: 0