Design the user experience around accessing served models via Seldon

[ca-scribner]

When a user serves a model using Seldon in their Kubeflow namespace, there are different use cases for accessing a model that they might want to satisfy:

1. accessing the model from a Kubeflow Notebook
2. accessing the model from a Kubeflow Pipeline (workaround demonstrated in #110, but a better solution may be possible)
3. accessing the model from another user's Kubeflow Notebook (eg userA serves the model and userB' Notebook consumes it)
4. accessing the model from another user's Kubeflow Pipeline (eg userA serves the model and userB's Pipeline consumes it)
5. accessing the model from outside Kubeflow with authentication required
6. accessing the model from outside Kubeflow without authentication required (eg: true public access to the model)

For each of these use cases, we should have:

• an automated test demonstrating it works
• a user guide showing a user how to use this feature

Closes canonical/bundle-kubeflow#557

[ca-scribner]

canonical/bundle-kubeflow#557 cites issues with this access story. In general, we need to define some clear manual or automated tests and see exactly what does and does not work, then provide guides for users.