Lustre/PNFS: refurbishes the :Lustre/pNFS configuration

This commit changes the way pNFS configuration is managed.
The pNFS configuration is no more a dedicated LUSTRE_PNFS block.
The configuration goes into a sub-block of FSAL dedicated configuration
block "LUSTRE"

The syntax becomes:
LUSTRE
{
	# fsalfsinfo related items
	xattr_access_rights = 0700;
	(...)

	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 ;
	    }
	  #more Dataservers if wantred
	  (...)
	}
}

Export specific configuration remain the same:
    EXPORT
    {
      (...)
      FSAL {
        name = "LUSTRE";
        PNFS
         {
            Stripe_Unit = 1048576;
            pnfs_enabled = true;
            DATASERVER
            {
                    DS_Id = 2 ;
            }
            DATASERVER
            {
                    DS_Id = 1 ;
            }
         }
      }
     (...)

IMPORTANT REMINDER:
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>
FSAL/pNFS: updates informaiton in  src/config_samples/config.txt

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