Allow three periodic tasks to hit slave

Allows the following periodic tasks to hit a slave
if a slave_connection is configured:

_run_pending_deletes
_instance_usage_audit
_poll_volume_usage

Blueprint: juno-slaveification
Change-Id: I38585562e3d3579299e32b5f9ced9311f672abcc
Merge "Allow three periodic tasks to hit slave"