[IMP] [WIP] mail: Set an OpenERP alias as the bounce address.

Attempts to fix issue #3347.

This makes OpenERP 7.0 issue a SMTP MAIL FROM command with an address it "owns".
Hmm, we should still use the `Return-Path` provided in the message as `smtp_from` (if there is one), otherwise it would be overriden totally by the "default bounce address", and that would prevent VERP.
The default bounce address should only be computed and used if there is no `Return-Path` header, so that we can try to use a better default than the `From` header.

For example:
```python
smtp_from = message['Return-Path']
if not smtp_from:
    smtp_from = self._get_default_bounce_address(cr, uid, context=context)
if not smtp_from:
    smtp_from = message['From']
```