[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).
[REF] mail: same_thread field changed into no_auto_thread, its contrary, to avoid migration issues (adding a 'always False' column is easier than an 'always True').
[FIX] module loading: manual x2x fields can now refer to manual models

The fix consists in this: when setting up models, ignore manual fields that
refer to unknown models if all models have not been loaded yet.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64