Added file option to generate_cluster_xml

When I used 0.90.1 I could create cluster.xml as below,

  python generate_cluster_xml.py myHostList.txt 20 > cluster.xml

But from 0.96, I had to edit all host name in cluster.xml. So I added file option. The file is supposed to have host name list or server's IP addresses.

Here is the usage.

  python generate_cluster_xml -f myHostList.txt -p 20 > cluster.xml