BasicConfig support for legacy values:

* A legacy value is a config section with a single-line.
* These values may be read from the BasicConfig interface so
  the deprecated Config class does not need to be exposed to
  clients.
* Made Config class more testable.

src/ripple/app/main/Application.cpp

@@ -887,7 +887,8 @@ public:
         {
             // VFALCO TODO Move all this into doSweep
 
-            boost::filesystem::space_info space = boost::filesystem::space (getConfig ().DATA_DIR);
+            boost::filesystem::space_info space =
+                    boost::filesystem::space (getConfig ().legacy ("database_path"));
 
             // VFALCO TODO Give this magic constant a name and move it into a well documented header
             //