fix #1347: let postfix fill in FQDN in fence mails

fix #1347: let postfix fill in FQDN in fence mails

Using the nodename in $mailto is not correct and can lead to mails not
forwarding in restrictive mail server configurations.
Also changes $mailfrom to 'root' instead of 'root@localhost', which
results in postfix appending the proper FQDN there, too. As a result the
Delivered-to header reads something like 'root@host.domain.tld' instead
of 'root@localhost', which is much more informational and more
consistent.

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