Create a new withExternalConfigFile pattern for property test cases

@mdeuser commented on [Mon Jul 25 2016](https://github.com/apache/incubator-openwhisk/issues/944)

Refactor test cases that make use of a separate property file to leverage a new withConfigFile pattern (similar to withAssetCleaner) to reduce the number of times we end up repeating create file, set env map, and delete file on error. This reduces the changes of forgetting a finally also.