Fix race in HBase regionserver startup vs ZK SASL authentication

Secure RegionServers sometimes fail to start with the following backtrace:

2012-03-22 17:20:16,737 FATAL org.apache.hadoop.hbase.regionserver.HRegionServer: ABORTING region server centos60-20.ent.cloudera.com,60020,1332462015929: Unexpected exception during initialization, aborting
org.apache.zookeeper.KeeperException$NoAuthException: KeeperErrorCode = NoAuth for /hbase/shutdown
at org.apache.zookeeper.KeeperException.create(KeeperException.java:113)
at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:1131)
at org.apache.hadoop.hbase.zookeeper.RecoverableZooKeeper.getData(RecoverableZooKeeper.java:295)
at org.apache.hadoop.hbase.zookeeper.ZKUtil.getDataInternal(ZKUtil.java:518)
at org.apache.hadoop.hbase.zookeeper.ZKUtil.getDataAndWatch(ZKUtil.java:494)
at org.apache.hadoop.hbase.zookeeper.ZooKeeperNodeTracker.start(ZooKeeperNodeTracker.java:77)
at org.apache.hadoop.hbase.regionserver.HRegionServer.initializeZooKeeper(HRegionServer.java:569)
at org.apache.hadoop.hbase.regionserver.HRegionServer.preRegistrationInitialization(HRegionServer.java:532)
at org.apache.hadoop.hbase.regionserver.HRegionServer.run(HRegionServer.java:634)
at java.lang.Thread.run(Thread.java:662)