pmxcfs: only exit parent when successfully started

pmxcfs: only exit parent when successfully started

since systemd depends that parent exits only
when the service is actually started, we need to wait for the
child to get to the point where it starts the fuse loop
and signal the parent to now exit and write the pid file

without this, we had an issue, where the
ExecStartPost hook (which runs pvecm updatecerts) did not run reliably,
but which is necessary to setup the nodes/ dir in /etc/pve
and generating the ssl certificates

this could also affect every service which has an
After=pve-cluster

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>