Having DATABASE_QUERY_LOGGING enabled results in devstack turning
on verbose mysql query logging. This results in a log file
/var/log/mariadb/mariadb-slow.log that grows to 10's of GB in
size in very little time (few weeks if that). Developers never
seem to notice this exists until their host OS runs out of disk
space due to this logfile that is never truncated.

Very few people will ever look at this logged data, so a more
sensible default is False, to make the out of the box setup be
suitable for the majority. Those few people who want to debug
mysql query performance can enable it in local.conf, as will
the devstack gate setup scripts.

Depends-On: I6970d61474528f554134d0aa333cd52b7b20f309
Change-Id: Ia4a366c839ac51623bc1fbee3560dc4d848cce14