Add email from partner by default to CRMLead

CHANGELOG:

### Fixed
- [#445](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/445) Set CRMLead `email_from` by default with partner's email.

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Fix from: https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/428 because of https://gitlab.com/coopdevs/otrs-somconnexio/-/merge_requests/101

Add email from partner by default to CRMLead

With OTRS-Somconnexio version 0.4.9, the email to create OTRS tickets is no longer
loaded from the partner (Customer) but its sevice (CRMLead).
Therefore, we need to assign a default value for CRMLead `email_from`.
If the lead is created with another email, this new one will overwrite
the default.