config.Add{Node,Instance}: get the ec id

This is ok because adding a node or instance cannot happen in a query.

We get the ec id from the LU and pass it to _EnsureUUID, which will
then for now not use it.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>