[Hadoop Wiki] Update of "AmazonEC2" by TomWhite

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The following page has been changed by TomWhite:
http://wiki.apache.org/hadoop/AmazonEC2

The comment on the change is:
Mention new S3 bucket for public AMIs.

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       * To see which versions are publicly available type: {{{
  % ec2-describe-images -x all | grep hadoop
  }}}
-      * The default value for `S3_BUCKET` (`hadoop-ec2-images`) is for public images. You normally only need to change this if you want to use a private image you have built yourself.      
+      * The default value for `S3_BUCKET` (`hadoop-ec2-images`) is for public images. Images for Hadoop version 0.17.1 and later are in the `hadoop-images` bucket, so you should change this variable if you want to use one of these images. You also need to change this if you want to use a private image you have built yourself.      
     * ('''Pre 0.17''') Hadoop cluster variables (`GROUP`, `MASTER_HOST`, `NO_INSTANCES`)
       * `GROUP` specifies the private group to run the cluster in. Typically the default value is fine.
       * `MASTER_HOST` is the hostname of the master node in the cluster. You need to set this to be a hostname that you have DNS control over - it needs resetting every time a cluster is launched. Services such as [http://www.dyndns.com/services/dns/dyndns/ DynDNS] and [http://developer.amazonwebservices.com/connect/thread.jspa?messageID=61609#61609 the like] make this fairly easy.