Fix secondary node selection for existing N+1

In case a secondary node is already N+1 failed, currently the node
selection will accept a node that cannot start (at all) the new instance
as valid. This is wrong, so we add a new simple check to prevent the
case of instance's memory size being higher than the node's free (not
available, which might be lower than 0 for N+1 failures) memory.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>