Use dib helper script to determine init system

diskimage-builder provides a helper script + env variable
to determine, which init system is used. We should use them
in elements instead of manual checks after update of raring
cloud images broke "if [ -d /etc/systemd/system ]" test.

Related-Bug: #1251610

Change-Id: I6d27491302ff0c4c6de82d6d60fc80e83fec519e