If the list of families does not change, their ports must not change, and it's
wrong to get this by relying on CPython implementation details. Even if we
automated the update of frontends with new urls, this couldn't be done
atomically and we'd get random failures.

Currently, frontends are only updated manually so we also want to minimize
changes when families are added/renamed/removed. By sorting alphabetically,
we have something predictable. Of course, this does not cover cases like the
following one:
- before: A, B, C
- after: A, C
Even if we added a 'port-base' parameter for the balancer, the port would
change for one of the 2 families.

We have no need for the moment, but we could go further with an optional list
parameter to choose the order, and a special value to skip ports. Another
option is to use publish-early but it's more complicated to implement and
we lose everything when we reinstanciate.

The sort in haproxy.cfg.in is for the stats page.