Zookeeper support

[benmccann]

I'd love to see Zookeeper support. Trying to decide between etcd and zookeeper. It'd be nice if we could use crypt in either case.

[kelseyhightower]

Zookeeper!!!!!

I have no zookeeper skills, but I'm sure we would accept a patch.

[bketelsen]

glad to accept a patch.

[jcftang]

Just for the record https://github.com/jcftang/crypt/blob/master/backend/zookeeper/zookeeper.go

I have get/set working relatively well, I'm not entirely sure of the list behaviour on how it really should be, it returns a list of kvp's and it doesnt deal the container znodes to well. The watch method is completely untested. I mainly added zk support as I am interested in getting viper to support zk for my needs.

This lifts a chunk of code from -- https://github.com/kelseyhightower/confd/blob/master/backends/zookeeper/client.go