Fix get/put when using dns_prefix

When a cluster is started with dns_prefix=True, the defaults in get/put still only look for master, not <dns_prefix>-master.

i.e. you get
$starcluster get mycluster /path/to/my/file .
StarCluster - (http://star.mit.edu/cluster) (v. 0.95.3)
...
!!! ERROR - node 'master' does not exist

Which can be easily handled with
$starcluster get mycluster -n mycluster-master  /path/to/my/file  .

but this fix is simple enough that I thought I'd send it.