Move _filter_non_model_columns method to CommonDbMixin
Move _filter_non_model_columns method to CommonDbMixin

Such way other non-core plugins may avoid duplicating this small
piece of code.

Change-Id: I00ae0b590fc2ad46d3667d18f29fccb5b40bde82

gkotton/vmware-nsx,gkotton/vmware-nsx