Add --config flag to specify location of json configuration file

Current implementation: The default config json file stays together with executable. In the case of containers based on open_etp_server_runtime image will stay in /usr/bin/ container's folder. The openETPServer will look for the .json (custom and default) in the following order: 1)Current working directory 2)Parent folder of the working dir (for pipeline tests) 3)/usr/bin

Need to add a --config optional flag. If given, this is the path of the json configuation file to use.