Added supervisor conf.d directory configuration

Supervisor allows configuration file includes. See http://supervisord.org/configuration.html

Therefore, let's add a basic supervisor.conf file under /etc/supervisor.conf which includes all files under /etc/supervisor/conf.d. This way Dockerfiles that extend FROM this base image can ADD a specific supervisor.conf file, such as jenkins-supervisor.conf or elasticsearch-supervisor.conf which will be picked up automatically when the supervisor daemon is run as a CMD.