Expand EntityReadMixin

Make `EntityReadMixin` provide the following methods to inheriting classes:

* `read`
* `read_json`

The `read` method acts just as before. The new `read_json` method does just what
you'd expect: read information from the server and return decoded JSON.

Add tests for the new `read_json` method.
I've expanded `read`'s signature. It now accepts an `attrs` arg in addition to the `auth` arg. This is slightly weird, but it allows for some very useful overrides. For example, see [here](https://github.com/Ichimonji10/robottelo/commit/4df2d175c41976f87af3e8aed5aab434c3815e41#diff-d41d8cd98f00b204e9800998ecf8427e).