Invoke method 'use' before returning configuration values

In order to ensure all actions applied by 'use()' are also
applied before returning a single configuration values.

Possible optimization: introduce a flag indicating that 'use' has
been called (not implemented for now).

src/com/sap/piper/ConfigurationHelper.groovy

@@ -103,6 +103,7 @@ class ConfigurationHelper implements Serializable {
     }
 
     def getConfigProperty(key) {
+        use()
         return getConfigPropertyNested(config, key)
     }