Rename the node storage commands

To reduce confusion, the following gnt-node commands are renamed:

- physical-volumes → list-storage
- modify-volume → modify-storage
- repair-volume → repair-storage

The NEWS file is update accordingly and it also gets emacs local
variables.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>