Read external promise parameters.

  The parameters should be provided by an external configuration file.
  Location of this configuration file is defined in the zope configuration
  file in a product_config named as the path of the ERP5 site.
  Example if the site id is erp5:
    <product-config /erp5>
      promise_path /tmp/promise.cfg
    </product-config>

  The promise configuration is a simple ConfigParser readable file (a list of
  section containing a list of string parameters.

  getPromiseParameter returns None if the parameter isn't found.