Added ability to specify user limits in role attributes

Added `node['ulimit']['users']` attribute to allow specify user limits in roles:

    "default_attributes": {
        "ulimit": {
            "users": {
                "tomcat": {
                    "filehandle_limit": 8193,
                    "process_limit": 61504
                },
                "hbase": {
                    "filehandle_limit": 32768
                }
            }
        }
    }