[COOK-3091] Check that Chef::Config[:config_file] exists before reading

Ticket: http://tickets.opscode.com/browse/COOK-3091

At least on some ChefSpec setups `Chef::Config[:config_file]` seems to
be `nil`, and then `IO.read` raises an exception from here.

Additionally, the recipe needs to check for file existence before trying
to read it.

Obsoletes: #10