run drop default constraint ahead of type

run drop default constraint ahead of type

Fixed issue in SQL Server dialect where the DROP that's automatically
emitted for existing default constraints during an ALTER COLUMN needs to
take place before not just the modification of the column's default, but
also before the column's type is changed.

also vendor sqlalchemy's metadata fixture which otherwise does not
integrate with alembic's version of the connection fixture

Fixes: #1744
Change-Id: Ia756da024297354449f53e9f3254820dec542bef