Chef Infra 18 deprecation warnings

[fatbasstard]

Hi,

When moving to Chef Infra 17 we get some deprecation warnings:

       Deprecation warnings that must be addressed before upgrading to Chef Infra 18:
       
         The  resource in the datadog cookbook should declare `unified_mode true` at 2 locations:
           - /tmp/kitchen/cache/cookbooks/datadog/resources/integration.rb
           - /tmp/kitchen/cache/cookbooks/datadog/resources/monitor.rb
          See https://docs.chef.io/deprecations_unified_mode/ for further details.

[axl89]

I just did a PR that should fix it 😄

#822