Add ability to graph entity 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`.
Mark required entity relationships in red
Reformat code
Clean up code using module `inspect`

The script now contains fewer lines of code and does not access __name__ class
members.
Merge pull request #962 from Ichimonji10/graph-entities

Add ability to graph entitiy relationships