Fix confd issue regarding --no-lvm-storage

If cluster is initialized with --no-lvm-storage then volume_group_name
does not exist in config.data. Thus we must define it as optional in
confd.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
[iustin@google.com: fixed Haskell RPC definition]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>