The model_base file is going to move to Neutron lib in
I2087c6f5f66154cdaa4d8fa3d86f5e493f1d24d9. This will mainly leave
behind only the standard attributes related stuff so the name
'model_base' won't make much sense. This moves the standard attribute
related things into its own module so model_base can conceivably be
eliminated entirely.
Change-Id: Icaf3220fbc5723f2b5421a494371ef274d7073c7