Refactor NodeManager.initialize

[jerroydmoore]

I was reading the node manager code and found optimizations and code readability improvements. They pass the unit tests.

• Renamed variables to increase code readability.
• Construct redis nodes outside of a nested loop for better performance.
• populate_start_nodes at the end of initialization.
• Skip keyslot loop if the cluster does not require full coverage.
• make_node_obj: Optionally remap nodes, and parse port as an int.