Add max_open_files to conf for debian

One may require the ability to adjust rabbitmq user open file limit.  On Debian/Ubuntu, the proper way is to use the conf file per the following: http://www.rabbitmq.com/install-debian.html#debian-system-limits

Added default 'max_open_files' attribute which is used in rabbitmq-env.conf if 'platform_family' is 'debian'