MXS-2483: Take SSLProvider into use

Servers and listeners now have a SSLProvider member variable that is used
for all SSL related tasks.

server/modules/authenticator/GSSAPI/GSSAPIBackendAuth/gssapi_backend_auth.cc

@@ -191,7 +191,7 @@ static bool gssapi_backend_auth_extract(DCB* dcb, GWBUF* buffer)
  */
 static bool gssapi_backend_auth_connectssl(DCB* dcb)
 {
-    return dcb->server->ssl_context() != NULL;
+    return dcb->server->ssl().context() != NULL;
 }
 
 /**