Use OS-aware calls to /etc/init.d/supervisor[d]

This patch fixes RHEL/CentOS/Fedora support by using the Puppet configuration variable from manifests/params.pp to call correct supervisord init script.

Previously, puppet-supervisor hardcoded ``/etc/init.d/supervisor`` as the init script but on RHEL/CentOS/Fedora the init script is actually called ``/etc/init.d/supervisord`` (note the trailing "d").