[FIX] Use _unknown model for missing models in relation fields

During a migration of database, it is possible that some custom field
("x_", state is 'manual') are relational to model from a module that is
not provided.

Note: this used to work in Odoo 7.0 but crashed in 8.0.

Closes #3877
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64