HMaster and HRegionServer should be able to login from Kerberos keytab when running on security-enabled Hadoop

Currently HBase can run on top of Hadoop 0.20 with security APIs, but only using the "simple" authentication method. There is currently no configuration or hooks for the HBase process to obtain Kerberos credentials so it can authenticate against secure Hadoop.

We should extend the current org.apache.hadoop.hbase.security.User hooks to allow obtaining credentials from a keytab file when security is enabled.