Only admin should be allowed to reconfig a cluster

ZOOKEEPER-107 introduces reconfiguration support via the reconfig() call. We should, at the very least, ensure that only the Admin can reconfigure a cluster. Perhaps restricting access to /zookeeper/config as well, though this is debatable. Surely one could ensure Admin only access via an ACL, but that would leave everyone who doesn't use ACLs unprotected. We could also force a default ACL to make it a bit more consistent (maybe).

Finally, making reconfig() only available to Admins means they have to run with zookeeper.DigestAuthenticationProvider.superDigest (which I am not sure if everyone does, or how would it work with other authentication providers).

Review board https://reviews.apache.org/r/51546/