[FIX] l10n_mx: add new tag for tax of 8 %

[GavyMG]

Add a new tag of 8 % to be used in the new Mexican tax for the northern border zone, and in the DIOT report.

Related to https://github.com/Vauxoo/enterprise/pull/495

---

The new tax reform of Mexico allows invoicing with a tax of 8 % in the northern border zone.

This reform consists in a reduction of 50% of the general rate (8% VAT), which will take effect on January 1, 2019. Those who can enjoy the stimulus are the following:

• Taxpayers with fiscal residence in the northern border area of the country who carry out commercial operations within said strip with products authorized for the stimulus.

[luistorresm]

@GavyMG

Please extend your PR description

[luistorresm]

@GavyMG

Please also create the tax for 8%.

Regards

[luistorresm]

Merged in Odoo