Improve a bit query API

Make QueryManager.query arguments optional.

At this point we are forced to speicify all filter, orderby, limit
arguments as well to save order. So for example if I would like to get
only 20 records I have to write something like:

ceilometer.query_sample.query(None, None, 20)

instead of

ceilometer.query_sample.query(limit=20)

As this doesn't break backward compatibility it shouldn't be a
big deal to merge it.

Change-Id: I5d589b27cafabd22204585a76f107125b2f3c7bd