MXS-2486: Move to_string into SSLProvider

The functionality is more a part of the provider than the context so it
should be defined in it. It also doesn't use any parts of the SSLContext
which makes it somewhat more clear that it doesn't belong there.

server/core/server.cc

@@ -524,9 +524,9 @@ void Server::print_to_dcb(DCB* dcb) const
                                                                 + server->stats.n_from_pool + 1);
         dcb_printf(dcb, "\tPool availability:                   %0.2lf%%\n", d * 100.0);
     }
-    if (server->ssl().context())
+    if (server->ssl().enabled())
     {
-        dcb_printf(dcb, "%s", server->ssl().context()->to_string().c_str());
+        dcb_printf(dcb, "%s", server->ssl().to_string().c_str());
     }
     if (server->proxy_protocol)
     {