Robotics Service Bus

Configurable configuration file list in rsb-java/src/main/java/rsb/util/ConfigLoader.java

fixes #2567

• rsb-java/src/main/java/rsb/util/ConfigLoader.java
  (ConfigLoader::CONFIG_FILES_VARIABLE): new constant; name of the
  environment variable that can be used to specify an alternate list of
  configuration files
  (ConfigLoader::CONFIG_FILE_KEY_SYSTEM): new constant; placeholder
  indicating the system-wide configuration file
  (ConfigLoader::CONFIG_FILE_KEY_PREFIX): similar for prefix
  (ConfigLoader::CONFIG_FILE_KEY_USER): similar for user
  (ConfigLoader::CONFIG_FILE_KEY_PWD): similar for current directory
  (ConfigLoader::DEFAULT_CONFIG_FILES): new constant; default config
  file cascades
  (ConfigLoader::ConfigLoader): compute config file cascades
  (ConfigLoader::computeConfigfileCascade): new method; compute config
  file cascade, taking into account CONFIG_FILES_VARIABLE
  (ConfigLoader::loadFiles): use configFileCascade
  (ConfigLoader::resolveAndLoadFile): new method; interpret config file
  placeholders, then call loadFileIfAvailable
  (ConfigLoader::loadFileIfAvailable): accept a description and print it
  in debug output
• rsb-java/src/test/java/rsb/Utilities.java
  (Utilities.createParticipantConfig): supply description to
  ConfigLoader.loadFileIfAvailable