Implement Discoverer class and lazy loading method for API resources (#…

…220)

* Remove resource container and implement Discoverer class

This will allow us to implement different strategies for discovering
API resources i.e. all requests at beginning, completely lazy,
background loading.

* Remove resource container unit test

* Add back line for ResourceList kind

* Add cache updating when resources are requested in __iter__ method

* Add back case where there is no match for _type field in JSONDecoder

* Change flag to for update cache to a field of Discoverer

* Add pytest unit test for discoverer

* Use generic get method for Discoverer subclasses

Only the logic for search needs to change between discoverers,
     so the get function is now defined in the Discoverer
     class instead of in each subclass

Signed-off-by: Fabian von Feilitzsch <fabian@fabianism.us>

* Update unit tests for discoverers

- Use request fixture to get the discoverer in the client fixture
- Minor style changes

Signed-off-by: Fabian von Feilitzsch <fabian@fabianism.us>