Use _pre_upgrade_### instead of _prerun_###

Other projects, including nova and oslo.db, use _pre_upgrade_### as the
function to run before a DB migration test. This is to differentiate it
from the downgrade tests.

Change-Id: I0da30f7c5a1c76a7ff0e735c36b753ef7a624db9
Merge "Use _pre_upgrade_### instead of _prerun_###"