make test_databases instance variable

make test_databases instance variable

eventually we'll need to do this to allow for testr testing, but in
doing so this also exposed that sqlite downgrade of source_volid
didn't work when data was in the table (sqlite just happily ignored
the drop, then failed when trying to added the column for a second
time during the snake walk.)

This fix also adds the correct sqlite downgrade path for migration
005 to actually drop source_volid, but ensure we don't loose any
other data in the process.

Change-Id: I082b53e108c4d564e33ef79979ea8c1642afdbcd