pg_upgradecluster and cluster_name

Hi,

As the cluster_name is managed by packaging, i think that
pg_upgradecluster should change cluster_name's value to match the new
version.

Currently, a new 9.6 instance created by pg_upgradecluster from 9.5
displays '9.5/main' in cluster_name, which become false.

regards,

--
Sébastien