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.

ala https://github.com/opscode/knife-ec2/commit/8a6fd7a3e3c96588070465f75b3254a7951b567d