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`.