Convert to new APIcast policy chain

[mikz]

APIcast 3.2 is going to have a policy chain - multiple policies chained together.

That means policies do not have to be aware of the 'apicast' module and multiple policies can be enabled at the same time.

The documentation is a bit scarce but your feedback on what is missing could be really helpful for us.
We've converted existing CORS example into a new policy: 3scale/APIcast#487

I'm happy to help how to make the setup so it is easy to test and develop.