ModelsMigrationsSync:Override compare_server_default

This test showed an error with alembic 0.6.6:
For MySQL Integer server_default value is compared incorrectly.
For example, when model has DefaultClause('0', for_update=False)
alembic finds it different from server variant "'0'".
For PostgreSQL Sting server_default value is compared incorrectly.
For example, when model has DefaultClause('', for_update=False)
alembic finds it different from server variant "''::character varying".

Such error appear in implementation of ModelsMigrationsSync to Neutron
https://review.openstack.org/76520.

Change-Id: I9730da1c0790a88f81570c51c36cc0cbe0651a5a