Enablement of unified_mode for v17+ Chef client compatibility

[ChefAustin]

(Shamelessly copypasta'd from facebook/IT-CPE#260)

Describe the Enhancement:

Converging the cookbooks in this repo on a v17+ Chef client result in the following deprecation warning being thrown (using cpe_activedirectory as an example):

  The cpe_activedirectory resource in the cpe_activedirectory cookbook should declare `unified_mode true` at 1 location:
    - /etc/chef/local-mode-cache/cache/cookbooks/cpe_activedirectory/resources/cpe_activedirectory.rb
   See https://docs.chef.io/deprecations_unified_mode/ for further details.

https://docs.chef.io/unified_mode/

Describe the Need:

Anyone using Chef client v17+ will face this deprecation warning until unified_mode true is appended to the first line of each custom resource file.

Current Alternative

Ignore the deprecation warning.

Can We Help You Implement This?:

Should be an easy PR (or series of PR's) to craft.

[ChefAustin]

Closed by #55