Generates the graph for traversal in convergence.
* Updates current traversal for the stack
* Deletes any sync_point entries of previous traversal
* Generates the graph for traversal based on
- resources loaded from db for the stack
- resources that exist in present template
* Stores resource.current_template_id and resource.requires
* Stores the edges of graph in stack.current_deps
* Creates sync_points for each node in graph
* Creates sync_point for stack.
blueprint convergence-prepare-traversal
Change-Id: I507e67b39c820ed46d3b269fc76d6cf18d0ef2d7