gnt-instance info: Return static info if node offline

Before this patch “gnt-instance info” would fail with the error message
“Error checking node $node: Node is marked offline” if the instance's
primary node is marked offline and the user didn't explicitely request
static information only. With this patch the LU will automatically
return static information if the instance's primary node is marked
offline.

Some explicit loops are changed to map().

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