warn about legacy supports fields in metadata

I.e.: Prevent users from writing `supports: linux` and similar. These are deprecated and will be removed. Also improve the warning to indicate what the user should do instead. Finally add tests to make sure we get all these.

lib/inspec/metadata.rb

@@ -73,7 +73,9 @@ module Inspec
         family = try_support[:'os-family']
         release = try_support[:release]
       elsif entry.is_a?(String)
-        @logger.warn("Using deprecated `supports` syntax: using `#{entry}` as OS family")
+        @logger.warn(
+          "Do not use deprecated `supports: #{entry}` syntax. Instead use "\
+          "`supports: {os-family: #{entry}}`.")
         family = entry
       end
 

test/unit/metadata_test.rb

@@ -6,6 +6,8 @@ require 'helper'
 require 'inspec/metadata'
 
 describe 'metadata with supported operating systems' do
+  let(:logger) { Minitest::Mock.new }
+
   describe 'running on ubuntu 14.04' do
     let (:backend) { MockLoader.new(:ubuntu1404).backend }
 
@@ -38,7 +40,7 @@ describe 'metadata with supported operating systems' do
     # @param [Type] params describe params
     # @return [Type] description of returned object
     def create_meta(params)
-      res = Inspec::Metadata.from_yaml('mock', "---", nil)
+      res = Inspec::Metadata.from_yaml('mock', "---", nil, logger)
       # manually inject supported parameters
       res.params[:supports] = params
       Inspec::Metadata.finalize(res, 'mock')
@@ -50,6 +52,16 @@ describe 'metadata with supported operating systems' do
       m.supports_transport?(backend).must_equal true
     end
 
+    it 'supports legacy simple support style, but warns' do
+      # i.e. setting this to something that would fail:
+      m = create_meta('linux')
+      logger.expect :warn, nil, [
+        'Do not use deprecated `supports: linux` '\
+        'syntax. Instead use `supports: {os-family: linux}`.']
+      m.supports_transport?(backend).must_equal true
+      logger.verify
+    end
+
     it 'loads a profile which supports os ubuntu' do
       m = create_meta({ 'os' => 'ubuntu' })
       m.supports_transport?(backend).must_equal true