Allow _poll_bandwidth_usage task to hit slave

Deployments that have a slave_connection configured can offload
read queries generated by this periodic task there. For
deployments with no slave, behavior remains the same.

Change-Id: Ibf27cdb94bc9b8c50066207c6285666cb29ebbf6
Blueprint: juno-slaveification
Merge "Allow _poll_bandwidth_usage task to hit slave"