Do not use a hardcoded Certificate Authority for https repositories

Do not use a hardcoded Certificate Authority for https repositories

Instead we will the use the CA certificate provided by the
ca-certificates packages, which is now a mandatory depency of
pve-manager since 067d24db982be44ff64cb5eb61a3297bba6afdc9 and
pve-manager 4.2-17. This change allows us in the future to
use different CA for our https repositories.

This changed has been tested OK with the following combination:
 * https repository using a StartCom certificate: works
 * https repository using a Let's encrypt certificate: works

User visible changes:
 * none : the new configuration file 75pveconf silently
 overwrites the olderone, except if local changes were made
 in which case you're presented with the traditional debian menu
 (keep local/ use packager version/ diff / open a shell)