Lustre/pNFS: splits the pNFS config in two parts.
Data Server definition is FSAL specific (stored in fsal_module)
Used DSs and Strip Unit is Export specific (stored in fsal_export)

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
Lustre/pNFS: updates src/config_samples/config.txt to speak about Lustre/pNFS options.

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