Add parallel Fabric support

This adds a `--parallel` option to fix that allows nodes to be configured in parallel. Required a bit of shifting around in how the node data bags are built and accessed as well as using Fabric's `execute` function instead of running chef.sync_node directly (I think there's probably a cleaner way to do this).

I don't really like the way the output works but don't really see any way to make it cleaner while still getting realtime feedback. Most useful change I could come up with was just re-enabling output_prefix when parallel is turned on.