Fix "Create a node" and "Add List Elements" APIs response code

Create a node and Add list elements APIs would return a 500 response
code when a JSON payload mismatching the Schema was sent as part of
the request body.

Updated the code to return response code 400.

Issue-ID: CPS-2319
Change-Id: I86d6af446966790d9e45045b330f0c411fcd0f1d
Signed-off-by: Arpit Singh <as00745003@techmahindra.com>
diff --git a/cps-rest/docs/openapi/components.yml b/cps-rest/docs/openapi/components.yml
index 99502e3..a2587eb 100644
--- a/cps-rest/docs/openapi/components.yml
+++ b/cps-rest/docs/openapi/components.yml
@@ -376,7 +376,7 @@
     Created:
       description: Created
       content:
-        text/plain:
+        application/json:
           schema:
             type: string
             example: my-resource