Fix downgrades in migration scripts 016 and 035

Downgrade behavior was not checked in Heat, so there are a few mistakes
in migration scripts. Fixed these scripts, set test attributes
`snake_walk` and `downgrade` to True due to allow migration testing and
avoid of regression.

Change-Id: I30cfb282cf7884d8b2748c6486a68d05072957af
Merge "Fix downgrades in migration scripts 016 and 035"