MINOR: ssl: Add traces to the switchctx callback

MINOR: ssl: Add traces to the switchctx callback

This callback allows to pick the used certificate on an SSL frontend.
The certificate selection is made according to the information sent by
the client in the clientHello. The traces that were added will allow to
better understand what certificate was chosen and why. It will also warn
us if the chosen certificate was the default one.
The actual certificate parsing happens in ssl_sock_chose_sni_ctx. It's
in this function that we actually get the filename of the certificate
used.