ModelsMigrationsSync:add correct server_default check for Enum

If Column which type is Enum has server_default it is compared
incorrectly for PostgreSQL.

For example, Column('test', sa.Enum('one', 'two', name='enum'),
                     server_default='one').
In model it has DefaultClause('one', for_update=False)) alembic
finds it different from server variant "'one'::enum".

Change-Id: Id30858f9c4c7a2bfb7f9313d47930845fe354a70
Merge "ModelsMigrationsSync:add correct server_default check for Enum"