Add XtreemFS driver to cinder

Add a new driver for xtreemfs.

How to use:

  - Install xtreemfs (Check: http://www.xtreemfs.org/download.php?t=source)

  - Check the quickstarts of how to setup (Check: http://www.xtreemfs.org/quickstart.php)

  - Create xtreemfs volumes:

     mkfs.xtreemfs <server-ip>/volume-0001
     mkfs.xtreemfs <server-ip>/volume-0002
     ....

  - Create a file /etc/cinder/shares.txt and write in it the create volumes:

      <server-ip>/volume-0001
      <server-ip>/volume-0002

  - Change cinder and nova configuration file accordantly to use XtreemFs.

More info can be found on the website:

http://www.xtreemfs.org/