Lustre/PNFS: repairs the pNFS feature for FSAL_LUSTRE

This commits repairs the previously lost pNFS feature for FSAL_LUSTRE.
All of the pNFS configuration is static and depicted in the config file.

The configuration is splitted in two parts:
- a FSAL_LUSTRE-wide configuration
- some export specific items.

FSAL_LUSTRE-wide configuration describes the usable Data server.
Example:
LUSTRE_PNFS
{
        DATASERVER
        {
                DS_Addr = ganesha_ds2.mynetwork.myhome ;
                DS_Port = 2049 ;
                DS_Id = 1 ;
        }
        DATASERVER
        {
                DS_Addr = ganesha_ds2.mynetwork.myhome ;
                DS_Port = 2049 ;
                DS_Id = 2 ;
        }
}

Export specific configuration in set like this:
EXPORT
{
  (...)
  FSAL {
    name = "LUSTRE";
    PNFS
     {
        Stripe_Unit = 1048576;
        pnfs_enabled = true;
        DATASERVER
        {
                DS_Id = 2 ;
        }
        DATASERVER
        {
                DS_Id = 1 ;
        }
     }
  }
 (...)

On each pNFS data server, a "pNFS aware" ganesha server should run.
pNFS can be enabled for some exports and disabled for others.
Different exports can use different sets of DSs.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>