[FIX] orm: do not drop foreign keys of transient models

During the update of a module, the existing foreign keys are dropped if they have a different ondelete_rule than the one specified on the field.
The foreign keys for many2one transiant -> non-transiant are created with cascade rule by default (see `m2o_add_foreign_key_checked` method) so the check needs to be realised in the same conditions.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64