Fix migration 016 for sqlite.

Sqlite doesn't support dropping a column from a table. The
downgrade path for migration 016 performed this. So this commit adds the
sql file to perform the downgrade manually.

Change-Id: I6aae263c3aab03a383c47fd29c46f0cc9b857549
Refactor data migration tests

This commit adds a more general framework for testing data
transitions at any db migration. Based on the test migration
framework in nova.

This commit also updates the existing individual migration
tests to use the new framework.

Change-Id: I917cfefe266a98f6c63613ac071af2dd676d67a1
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp