rc-local and rabbitmq-server ordering.

This patch will configure the startup ordering dependency between the
rc-local.service and rabbitmq-server.service. If not configured systemd
will start both services simultaneously and it might cause the first boot
execution to fail because rabbitmq-server wasn't up and running when the
scripts are running. Before I8985d8020f94c257d426065cf914213b35ec959f
this problem wasn't apparent because rabbitmq was manually restarted at
the beginning of the first-boot.d/98-boot-stack script.

Change-Id: Id86b43036f448262f241c04369191dfa253a6aeb