update SSL logic to accomodate pregenerated certificates
update SSL logic to accomodate pregenerated certificates

The @ecobytes instance showed a misbehaviour concerning TLS certificates which had been set in instance.cfg.
Ultimately 89cdc314ea9ad93d7951d1800e525f85ec47eaac#diff-c226f84131b50059e044a64c41fe782c added a logic which would not account for preconfigured TLS certificates anymore.
This commit introduces a small change to revive usage of third-party certificates without overwriting their configuration options.

modoboa/modoboa-installer,modoboa/modoboa-installer