Hitoshi Mitake d8888ded12 *: support deleting user in v3 auth
This commit adds a functionality of user deletion. It can be invoked
with the new user delete command.

Example usage:
$ ETCDCTL_API=3 etcdctl user delete usr1
2016-03-31 13:18:51 +09:00
..
2016-03-31 13:18:51 +09:00