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.
Merge pull request #1259 from Ichimonji10/entity-read-mixin

Add class robottelo.orm.EntityReadMixin