Ticket #96: tuning.conf supports 'router' as node type.

The official list of supported keywords is:
 MGS, MDS, OSS, CLIENT, CTL, ROUTER, RTR

Example file has been updated to reflect that.

This is also a big cleaning in TuningModel.
Ticket #161: Drop code managing FS in backend.

Remove all set_status_fs_xxx() method and associated code.
Ticket #125: Allow to unregister a FS even with error.

If some nodes are unreachable when removing a filesystem, we
now allow the operation to continue and unregister the filesystem
from the backend. It is user's reponsability to fix the errors.