InstanceInfo RPC call

This patch implements single instance info call - somewhat similar to
all_instances_info, except we give a specific instance name.

Current implementation of reading the InstanceInfo value is somewhat
counter-intuitive because when we query a node on thich there is
no instance with such name, this is not considered an error - but
it does produce an empty answer. Therefore the resulting type is
optional.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>