partially fix #2825: authkey: rotate if it was generated in the future

partially fix #2825: authkey: rotate if it was generated in the future

Can happen if the RTC is in the future during installation and first
boot, when during key generation the clock is in the future and then,
after the key was already generated, jumps back in time.

Allow a fuzz of $auth_graceperiod, which is currently 5 minutes, as
that fuzz allows some minor, not really problematic, time sync
disparity in clusters.

If an old authkey exists, meaning we rotated at least once, check it's
time too. Only rotate if it'd not be valid for any tickets in the
cluster anymore, i.e., if it difference between the current key is >
$ticket_lifetime (2 hours)..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>