ENH: Option to prefix dns name of clusters with the tag name.  ie master on mycluster --> mycluster-master.

Hi,  

This PR adds the option to prefix dns hostnames with the cluster tag.  I wrote about this a little while ago on the mailing list and finally had some time today to do this.   http://star.mit.edu/cluster/mlarchives/1820.html

The PR does not change any existing functionality, and adds an option to prefix host names.  Specifically, 

    If you have a cluster named "mycluster" and set the option the prefix dns names with the cluster tag, the new naming scheme would be:
       master --> mycluster-master
       node001 --> mycluster-node001 

Please take a look and let me know what you think!  I also started 0.95 in the Changelog.

Alex