[FIX] mrp: better selection of bom

When searching for a bom based on a product.product, the method used to return any bom linked to the template (so searching for bom for variant A could return bom for variant B if both share the same product.template).
This will allow to return bom not linked to a variant without returning the one of another variant (opw 609358)
[FIX] web_kanban: parent can be undefined in some cases

opw-611106, when clicking quickly Sales > Opportunities than Sales > Leads.
[FIX] purchase: now allow 'can be purchased' to be checked even if not variant

Before, the field purchase_ok was set as readonly on the product form if the product wasn't a variant, to avoid users to set all the products variants associated to the product template to be set as purchase able in one action.

We now consider that this can be done by the end user
[MERGE] forward port of branch saas-5 up to 7d7f6fe
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64