using ::Config attributes raises stack error

[wrightp]

In my code I did something like this:

default :configuration, 'fun value!'

I was using "configuration" in a different context (I wasn't trying to override Mixlib-config's intent since I didn't know at the time :configuration was an attribute), and I got this error:

Failure/Error: Unable to find matching line from backtrace
SystemStackError:
stack level too deep
# ./vendor/bundle/ruby/2.1.0/gems/mixlib-config-2.1.0/lib/mixlib/config.rb:444

It would be useful if mixlib-config raised an error or warning that I was trying to use the ::Config accessors (https://github.com/opscode/mixlib-config/blob/master/lib/mixlib/config.rb#L30-L33).

[lamont-granquist]

This should probably be something like 'avoid letting users try to stomp the methods on Chef::Config' and before allowing a config to be declared it should check responds_to? first.