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.
No, the above wouldn't be valid. `SomeEntity(id=N).read().other_entity` is an instance of `other_entity`.