Add maybePrintInsts for the instance listing

This again abstracts a bit the instance listing. Due to the fact that
I don't want to import Cluster.hs in CLI.hs, we pass the already
generated output. It also moves the instance display to stderr.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>