Additional context for UnknownConfigOptionError error message

[tano]

Describe the Enhancement:

Having the same values for different keys in config may lead to ambiguous error message in the following function: https://github.com/chef/mixlib-config/blob/7e682cd35ce94daa43b65425d83628a27689df3a/lib/mixlib/config.rb#L626.

So maybe some additional context should be added here to clarify the error message in case of the presence of same values across the config.