lxc-start started to default to daemonize the container when starting
this conflicts with type=simple of the systemd unit

call lxc-start with -F and thus force execution in foreground
that way we can feed the log to journald properly and keep type=simple

Debian-Bug: https://bugs.debian.org/826100Signed-off-by: Evgeni Golov <evgeni@golov.de>