MINOR: ssl: load certificates in alphabetical order

MINOR: ssl: load certificates in alphabetical order

As reported by Raphaël Enrici, certificates loaded from a directory are loaded
in a non predictive order. If no certificate was first loaded from a file, it
can result in different behaviours when haproxy is used in cluster.
We can also imagine other cases which weren't met yet.

Instead of using readdir(), we can use scandir() and sort files alphabetically.
This will ensure a predictive behaviour.

This patch should also be backported to 1.5.