Fix robottelo.api.client

The Python requests library has changed upstream. Make module
`robottelo.api.client` match the upstream changes.

Update the tests for this module. Use the mock library. This allows mock classes
to be dropped, manual patching/unpatching to be dropped, and function calls to
be inspected at a more granual level.
Merge pull request #1505 from Ichimonji10/fix-api-client

Fix robottelo.api.client
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`.