Add method `EntityReadMixin.read_raw`

`read_raw` makes a call to the server being tested and returns a
`requests.models.Response` object. It does not perform any validation, such as
checking the HTTP response code or decoding JSON.

Make `EntityReadMixin.read_json` call `EntityReadMixin.read_json`.
Remove 404 handling logic from `EntityReadMixin.read_json` and add this logic to
class `ActivationKey`. The 404 handling logic is only relevant to activation
keys.

Make module `test_multiple_paths` call `read_raw` instead of directly calling
`robottelo.api.client`.
At least one of the tests in module `test_multiple_paths` passes an empty tuple of credentials to `read_raw`, thus forcing the client to perform an unauthenticated request to the server.