[Bug] Use DomainReadOnlyUser from the input provided in config

* Making the function case-sensitive
* making the function stricter as `cn` can be part of username

Author: Himani Anil Deshpande

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

@@ -21,7 +21,9 @@ 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"
+    ['cn=', 'CN='].each do |param_search|
+      return token if token.include? param_search
+    end
   end
   ""
 end