[Bug] Use DomainReadOnlyUser from the input provided in config

* Making the function case-sensitive and strickter as `cn` can be part of user-name too

Author: Himani Anil Deshpande

cookbooks/aws-parallelcluster-environment/libraries/directory_service_utils.rb

@@ -21,7 +21,7 @@ def parse_arn(arn_string)
 def _get_cn_subparam(param)
   tokens = param.split(",") unless param.blank?
   tokens.each do |token|
-    return token if token.include? "cn"
+    return token if token.downcase.start_with? "cn"
   end
   ""
 end