Revert "Change init system to systemd"

This reverts commit 9d69c4ce67b6f7dba4f81243585bc1c8b12554f8.

While systemd works, but there are quite a few issues with the
*installed* machine:
* does not start quite a few essential services like login-manager
* sshd, network-manager fails to start
* ... a few others.

Therefore I conclude that for LIVE systems systemd on wheezy is not yet
ready and reverting to sysvinit by default.