Add ability to graph entitiy relationships

Add a script at `scripts/graph_entities.py`. This standalone script inspects the
relationships between the entities in module `robottelo.entities` and outputs
relationshp information to stdout in DOT format.

Add command `graph-entities` to the make file. The command executes the new
script, pipes into graphviz and generates an SVG image, all in one go. Make git
ignore the image generated.

Add "graphviz" to `requirements-optional.txt`.
The inspect module can make the code simpler:

```python
>>> import inspect
>>> from robottelo import entities
>>> members = [member for member in inspect.getmembers(entities, inspect.isclass) if issubclass(member[1], orm.Entity)]
>>> for entity_name, entity in members:
...     print entity_name, entity
... 
# output omitted
```