Rework the required_nodes field

This was used to verify the result of instance allocation. However, this
is not logic which belongs to the cmdlib rather than to the iallocator
request itself. So abstract it by its own function. And update the
ValidateResult accordingly.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>