Add redis attribute to set ulimit

The ulimit for redis can be set in /etc/default/redis-server. This adds an attribute, ulimit, that sets the ulimit setting in that file. If the attribute is unset or empty, the ulimit setting in /etc/default/redis-server file is commented out, which is how the file ships by default.

Changing the ulimit causes the redis-server to restart.

The default ulimit may be too low to handle a large number of clients. See http://redis.io/topics/clients for more information.