Improve logging of sql queries a bit.

Improve logging of sql queries a bit.

Docs:
  http://docs.sqlalchemy.org/en/latest/core/engines.html#configuring-logging

So for an application utilizing python logging for real
(and MediaGoblin should) the rule is:
- Don't use echo=True,
- but reconfigure the appropiate loggers' level.

So replaced the echo=True by a line to reconfigure the
appropiate logger to achieve the same effect.

This still dumps whole bloats of SQL queries into the main
log, but at least they're not duped any more.