This change 'rehomes' model_base.py from neutron.
This is not quite a verbatim copy due to the following differences:
- The field sizes for columns are replaced with the new constants
from neutron_lib/db/constants.py
- The tenant_id deprecation message is adjusted slightly.
- The StandardAttribute table is not included.
We also add some base classes and fixtures for DB unit tests.
Partially Implements: Blueprint neutron-lib
Change-Id: I2087c6f5f66154cdaa4d8fa3d86f5e493f1d24d9
8be88da5c1