[JUJU-3999] Avoid parsing endpoint for overlay offers

[cderici]

Description

This is an addendum PR on #817, that attempts to fix #816.

So the main thing here is that we don't accept endpoints that are not in "<application-name>:<endpoint-name>[,...]" format, like:

 $ j offer grafana
ERROR endpoints must conform to format "<application-name>:<endpoint-name>[,...]"

unless, it's coming from a bundle overlay, such as:

description: Another overlay to create an offer
applications:
  grafana:
    offers:
      dashboards:
        endpoints:
          - dashboards

Pylibjuju already has the necessary mechanism that passes along the application name and offer name etc from a bundle/overlay (here), this PR is a small change that adjusts the controller.create_offer (that actually calls the ApplicationOffersFacade.Offer) to not parse the endpoint to get the offer_name and application_name (that the function parameters are overriding anyways).

Fixes #816

QA Steps

#817 already adds a test that demonstrates the error, this PR extends that to also check for the offer in the model. So the following should pass:

tox -e integration -- tests/integration/test_model.py::test_deploy_bundle_with_multiple_overlays_with_include_files

[cderici]

/merge