Add class robottelo.orm.EntityReadMixin

Create a mixin class named `EntityReadMixin`. This mixin adds a method named
`read` to classes that use it. The `read` method returns an entity object whose
attributes have been populated using info fetched from Foreman.

Add a test for this new mixin.
This is an interesting idea. It's totally possible to do something clever and cool with the proxy design pattern here. For example:

```python
entity = SomeEntity(id=N).read()
entity.related_entity.id  # returns the object's ID immediately
entity.related_entity.other_attr  # causes related_entity to read in all attributes
```