ViperizeFlags checks whether a configuration file was specified, reads it, and updates the configuration variables in the specified flag set accordingly. Must be called after framework.HandleFlags() and before framework.AfterReadingAllFlags().
The logic is so that a required configuration file must be present. If empty, the optional configuration file is used instead, unless also empty.
Files can be specified with just a base name ("e2e", matches "e2e.json/yaml/..." in the current directory) or with path and suffix.