Move patch from oslo to drop unique constraints with sqlite

oslo-incubator commit 3f503faac for making sqlite work with dropping
unique constraints in database migrations. This was made in
oslo-incubator since at the time sqlalchemy-migrate was not in
stackforge. Now that we can update sqlalchemy-migrate, move the patch
over from oslo.

This change also adds the support for the case that a unique constraint
is dropped because the column it's on is dropped.

Note that there are already unit tests that cover dropping a unique
constraint directly and implicitly via dropping a column that is in
the unique constraint.

Related-Bug: #1307266

Change-Id: I5ee8082a83aebf66f6e1dacb093ed79e13f73f5e