Add Command class. Allow extra commands for node create.

Adds a Command class to hold commands plus some helpers for adding new command instances. The Command class allows for some extra information to be held with the command, in this case, whether the command is allowed to fail.

Also added is a new key for nodes to allow for extra commands to be run after the node creation. This is useful for nodes that may require multiple chef runs to fully converge, or if any other commands need to be run after the node is created.