Update sqlalchemy migrate scripts for postgres

pgsql <= 8.3 was lax about char->other casting but this was tightened
up in 8.4+. We must now specify the USING clause for such a cast to
succeed. The SQL queries generated by sqlalchemy for pgsql don't
support this, so an override is needed to support pgsql for the trove
migrate scripts.

Closes-bug: 1293465
Change-Id: I217fc861036911e61bafe706e338bb3b8f49e708
Merge "Update sqlalchemy migrate scripts for postgres"