Register region server in zookeeper by ip address

HBase cluster isn't always setup along with a DNS server. But regionservers now register their hostnames in zookeeper, which bring some inconvenience when regionserver isn't in one DNS server. In such situation, clients have to maintain the ip/hostname mapping in their /etc/hosts files in order to resolve the hostname returned from zookeeper to the right address.

However, this causes a lot of pain for clients to maintain the mapping, especially when adding new machines to the cluster, or some machines' address changed due to some reason. All clients need to update their host mapping files.

The issue is to address this problem above, and try to add an option to let each regionserver record themself by ip address, instead of hostname only.