Periodic task poll_bandwidth_usage can use slave

Host instance lists will be retrieved from a slave
if the deployment has one configured. If no slave
 is configured behavoir remains the same.

There is still some more slaveification that can be
done with the bw_usage_cache queries. I've made notes
and also made a few changes to the db_api so that these
can be used when we get the necesssary bw_usage_cache
object created.

Change-Id: I4286f545573bf81cb036c29d48afa39a14cfad61
Partially-implments: bp-periodic-tasks-to-slave
Merge "Periodic task poll_bandwidth_usage can use slave"