Add Support to mimic CDN providers

This PR adds support to mimic CDN provider API calls. While this might not fit in the scope of Mimic (since this does not mock Openstack endpoints, but is aimed at mocking Fastly API calls), the intent of this PR is to get feedback on if this is the right approach. If this turns out to fit into the Mimic context & can be merged, I can clean up this PR & add tests.
More generally, to manage top-level state associated with a service, you should just have objects.  Every function in this file which takes a `fastly_cache` should be put together onto a class with methods that manipulate state on `self`.