We need to set this in the settings.json.
While we're there, set the defaults in my.cnf. I don't think that
actually matters, but it also shouldn't hurt.
Change-Id: I52e65136383f6c06d1693dd3d396f527c7e0b377
Upstream likes building the settings file into the image, but that's
less exciting, let's bind-mount ours in.
Depends-On: https://review.opendev.org/717491/
Change-Id: Ia1894d884ef2a84e1282345b77fe07bf8898f367