[FIX] models: fields_get() shall not return info about fields not set up yet

When processing data files during a module installation/upgrade, not all fields
are set up yet, in particular relational custom fields.  Make fields_get()
ignore those fields, so that views can be created/updated and validated,
provided they do not refer to those fields...
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64