manager: send notifications via new notification module

manager: send notifications via new notification module

... instead of using sendmail directly.

If the new 'notify.target-fencing' parameter from datacenter config
is set, we use it as a target for notifications. If it is not set,
we send the notification to the default target (mail-to-root).

There is also a new 'notify.fencing' paramter which controls if
notifications should be sent at all. If it is not set, we
default to the old behavior, which is to send.

Also add dependency to the `libpve-notify-perl` package to d/control.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>