Update response codes

Change 202 response code to 200/201/204 since our APIs are not asynchronous.

Summary:

• CreateGroup -> "201 Created"
• ListGroup -> "200 OK"
• DeleteGroup -> "204 No Content"
• ListMember -> "200 OK"
• AddMember -> "200 OK"
• RemoveMember -> "204 No Content"
• UpdateGroup -> "200 OK"