Validate outermost request body element name consistently

Validate outermost request body element name consistently

1. Return useful error message instead of empty error message.
ex.- Return "Missing required element '<element name>' in request body"
     instead of "Bad Request"
2. Return consistent error message in case request body element
name is not present.
3. Added 'assert_valid_body' method to raise HTTPBadRequest exception
if request body is not valid. It is not possible to modify
'is_valid_body' method itself to raise exception because V1 and v2
api's return different error code when 'is_valid_body' returns False.

APIImpact

Closes-bug: #1461402
Change-Id: I81764cd8517484fbee4beae2b30668c8b180d677