Give ability to define constants that may be used throught the configuration

OpenSim/Server/Base/ServicesServerBase.cs

@@ -129,6 +129,8 @@ namespace OpenSim.Server.Base
             // Merge the configuration from the command line into the loaded file
             Config.Merge(argvConfig);
 
+            Config.ReplaceKeyValues();
+
             // Refresh the startupConfig post merge
             if (Config.Configs["Startup"] != null)
             {