Module update does not overwrite existing therms when i18n-overwrite is set

[Yann Papouin]

If the server is started with the "--i18n-overwrite" option, it is entirely ignored when updating module in these two cases:
- update from command line
- update from module view

In "tools/config.py", following is stated:
"the i18n-overwrite option cannot be used without the i18n-import option or without the update option"

I made a fix in _load_data in "modules/loading.py" where a context is created with the overwite option (was None before):
# Update translations for all installed languages
modobj.update_translations(cr, SUPERUSER_ID, [module_id], None, {'overwrite': openerp.tools.config["overwrite_existing_translations"]})