AWS CloudFormation Update: Added support for UNKNOWN drift status.

Author: AWS

.changes/next-release/feature-AWSCloudFormation-ce634db.json

@@ -0,0 +1,6 @@
+{
+    "type": "feature",
+    "category": "AWS CloudFormation",
+    "contributor": "",
+    "description": "Added support for UNKNOWN drift status."
+}