Fix explicit server allocation

The test cases allocated servers in a way that doesn't comfortably suit
the way the servers are now allocated. Adding a helper C++ class to load
module defaults makes it easier to do explicit server initialization in
tests.

The binlogrouter was also fixed in this commit as it uses servers much
like a test would use.

include/maxscale/config.hh

@@ -0,0 +1,38 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/config.h>
+
+namespace maxscale
+{
+
+// Helper class for allocating temporary configuration parameters
+class ParamList
+{
+public:
+    ParamList(const ParamList&) = delete;
+    ParamList& operator=(const ParamList&) = delete;
+
+    ParamList(std::initializer_list<std::pair<const char*, const char*>> list,
+              const MXS_MODULE_PARAM* module_params = NULL);
+
+    ~ParamList();
+
+    MXS_CONFIG_PARAMETER* params();
+
+private:
+    CONFIG_CONTEXT m_ctx = {(char*)""};
+};
+
+}