API unit test base

There are currently no or few tests for the API - and by tests, I mean being able to simulate API calls and observe the results. It's important that we're able to write them; there is currently a very high barrier because there's a lot of setup that needs to happen. This blueprint will implement enough of a base class that it's possible to test against the API by sending requests and observing RPC calls generated and database changes made.

I believe heat and glance both use in-memory databases for tests rather than mocks, and this had support from the murano dev team. The base tests should set up and migrate a test database, and provide the ability to create admin and non-admin contexts to test with. They should also mock the RPC library to test that events are generated correctly.