Ability to explicitly include and exclude hosts from cluster

Both MR and HDFS (see http://hadoop.apache.org/common/docs/r0.20.0/cluster_setup.html; look for "dfs.hosts", "dfs.hosts.exclude", "mapred.hosts", "mapred.hosts.exclude") provide the user a way to deny certain slave daemons from joining the cluster. The use for this is two-fold: it prevents developers with a client configuration from joining the cluster from their laptop on accident, and it provides a mechanism to explicitly decommission a node while it's in repair or what-not.

A similar explicit cluster membership would be useful for HBase, for the same reasons. Just yesterday a user found out that his organization was running regionservers on machines that weren't supposed to be running regionservers. It wasn't a huge deal, but this feature would make it easier for this user to prevent the co-admins from making this mistake.

I'd note that HDFS and MR are inconsistent at the moment as to whether their exclude files should have IPs, hostnames, or both. Clearly defining and documenting that is useful.