Refactored application installers to use generic download file names.

- It is no longer necessary to specify an *_APP_FILE key in the
  settings.sh. A generic file name for the download is used instead
  and controlled by the installer functions.
- This simplifies the settings.sh by removing app file keys that
  didn't provide any value and caused unnecessary setup and confusion.
Refactored install_tar_app function so that decompression options are,
in fact, optional.