ZODB Components: Properly handle addition of template_* properties in BusinessTemplate PropertySheet.

On the plus side, this avoids an ugly 'except AttributeError: pass' which does
not work anyway with the scenario below.

Assuming the following:
  * template_XXX Property (accessor: getTemplateXXX) recently added to BusinessTemplate PropertySheet.
  * erp5_YYY sets template_A Property (erp5_YYY/bt/template_XXX.xml).
  This commit handles the following:
    1. Stop instance with old ERP5 without template_XXX.
    2. Update erp5.git.
    3. Start instance.
    4. Upgrade erp5_property_sheets and erp5_YYY bt5s in *one* transaction from portal_templates UI.
    => When upgrading erp5_YYY, BusinessTemplate.importFile() imports erp5_YYY/bt/*.xml where the
       list of files is based on BusinessTemplate class propertyMap(), but at this point accessors
       have not been re-generated yet, thus template_XXX is not returned by propertyMap() and
       erp5_YYY/bt/template_XXX.xml is not imported.
       => portal_templates/erp5_YYY new BT does not have template_XXX property set at all.