[IMP] refactor account_partner_required

* extention points for policies
* use constraints for readbility and robustness
[IMP] account_partner_required: make test use [ids] for write

Some other modules such as account_constraints require it.
[IMP] .travis.yml account_partner_required test now works around account_constraints

Therefore we can lighten the EXCLUDE/INCLUDE dance.

Signed-off-by: Stéphane Bidoul <stephane.bidoul@acsone.eu>
[FIX] account_partner_required: constraint depends on account_id
[FIX] pep8
[FIX] account_partner_required: constraint also depends on debit and credit
[FIX] account_partner_required: make policies translatable
Merge pull request #48 from acsone/7.0-account_partner_required_refactor-sbi

7.0 refactor account_partner_required