Normalize VAT number

We want to normalize the VAT numbers in our system following the next rules:

• Remove dots (.), dashes (-) and spaces.
• If is a Customer must have the ES prefix. Only customers. We need to create providers from other countries.

We also use the base_vat module which use the ckech_vat_es method of vatnumber Python lib, but this validation depends on vatand country_id fields.

Changing to required the vat and country_id fields in the ResPartner form force this validation.

We adapt the next models:

• SubscriptionRequest: In validation method, we convert the vat received in a normalized spanish vat (ES prefix).
• ResPartner: In create/write methods we normalize the VAT.
• Also in the ResPartner search of the REST API we normalize the VAT received to improve the search.