Don't save mysql logs

Currently a full tempest run produces 600MiB of _compressed_ log data,
of which 324MiB are mysql logs.  This is because devstack configures
the mysql slow query log to log every query.  This seems like an
excessive level of debugging for the gate, so lets not save those
logs.

Change-Id: Ib4cdfc929b93b289916295ff365a46979555a574
Merge "Don't save mysql logs"