Fix the default prototype for join_group

Otherwise it's different than the one used in the ZooKeeper
implementation.

Change-Id: I1ba3af04756a6b27a6d09c8c7b76a213ec9bf359
tests: fix variable name

Change-Id: Ie3c3b4d4cd871fa9b03225a193b439e1cfc18674
Signed-off-by: Julien Danjou <julien@danjou.info>