allow the checkIfFileExists method to check if 1 or more files exist

This fix changes the file checking to check for more than 1 file.
Thus the file element of the test config file can either be a list of files or a single file

i.e 
    "file":"/usr/local/kafkaoffset/start",
or
    "file":["/usr/local/kafkaoffset/start","/usr/local/kafkaoffset/kafka-offset-monitor.jar"]