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_installer/ssl.py

@@ -11,6 +11,7 @@ class CertificateBackend(object):
     def __init__(self, config):
         """Set path to certificates."""
         self.config = config
+        if not config.has_option("general", "tls_key_file"):
             for base_dir in ["/etc/pki/tls", "/etc/ssl"]:
                 if os.path.exists(base_dir):
                     self.config.set(
@@ -21,6 +22,8 @@ class CertificateBackend(object):
                         "{}/certs/%(hostname)s.cert".format(base_dir))
                     return
             raise RuntimeError("Cannot find a directory to store certificate")
+        else:
+            return
 
 
 class SelfSignedCertificate(CertificateBackend):