systemd service: wait for networking, don't daemonize

This change updates the systemd elasticsearch service configuration to depend
on boot-time networking being available before ES is started (see
http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/ for a
discussion of the specific network-online target used in the service file).

Also, the ES script does not fork / daemonize when started by systemd since
this is the recommended behaviour for systemd services (see
http://0pointer.de/blog/projects/systemd-for-admins-3.html).

fixes #8636

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>