Adds warning when default --attribute is in effect

A common pitfall for new or intermediate users of Chef is invoking "knife ssh ..." without specifying the crucial "-a or --attribute" flag/option. When -a is not explicitly specified, node["fqdn"] is used to establish ssh connections. The node's "fqdn" attribute may or may not be a navigable hostname(s). Some fall into the pit and some never know they need -a.