cluster join: ensure updatecerts gets called on quorate cluster

cluster join: ensure updatecerts gets called on quorate cluster

We moved the start of pve-cluster together with the one of corosync
earlier, before the quorate check.
This meant that the 'pvecm updatecerts --silent' we call in the
from the pve-cluster.service through ExecStartPost exited as it has
not yet quorum.

So factor the respective code out to the Cluster perl module and
call this function manually after we reached quorum.

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