Run this command. Commands take a ZooKeeperServer and String-valued
keyword arguments and return a map containing any information
constituting the response to the command. Commands are responsible for
parsing keyword arguments and performing any error handling if necessary.
Errors should be reported by setting the "error" entry of the returned
map with an appropriate message rather than throwing an exception.
kwargs - keyword -> argument value mapping
Map representing response to command containing at minimum:
- "command" key containing the command's primary name
- "error" key containing a String error message or null if no error