[FIX] procurement: missing variable

scheduler_cron_id was not initialised so the scheduler always crashed (with just info message)
[FIX] mrp: duplicated find bom buttons

The product_id field is not required on a mrp.bom (while product_tmpl_id is). When searching for BOMs linked to a product.product, we should display the one specific to this product and the ones linked to the template (but not linked to another variant).