Rename get_graph() -> execution_graph

Instead of having a generic get_graph()
it is nicer to have this function be named
closer to what it returns (the graph of tasks
to be executed) as well as make it a property
since it is more of a property of the engine.

Change-Id: I4bb458b1069d7e6877f1cc51d42bfc0d8c751951
Merge "Rename get_graph() -> execution_graph"