Fix the `graph-entities` make rule

The `robottelo.factories` module no longer exists. Roughly equivalent
functionality has been moded into the `robottelo.orm` module.

    $ make graph-entities
    scripts/graph_entities.py | dot -Tsvg -o entities.svg
    $ echo $?
    0
Merge pull request #1641 from Ichimonji10/graph-entities

Fix the `graph-entities` make rule