use config file settings to configure database

Let the database configuration scripts get the configuration parameter from the director config file.
Now, the scripts try to get there values in following order:
  1. environment variable (db_name, db_user, db_password)
  2. director config file
  3. fallback value (from configure)

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>