Configsets API should allow update of existing configset

With the Configsets API it is not possible to update a configset once it has been created: the UPLOAD action will throw an exception "The configuration X already exists in zookeeper".  This is different behavior than the other mechanisms that allow uploading either a new or updated configuration to Zookeeper.

For someone trying to update a configuration, the only path given through the APIs is to delete the associated collection and configuration and start over from scratch. That, or a one-off invocation of the zk local scripts when these APIs are far superior for remote management .

Thanks,
Alex