Periodic task _heal_instance_info_cache can now use slave db

This periodic task will now use a slave db for all reads if a slave
is configured. For deployments with no slave, behavior remains unchanged.

Change-Id: Iafda7ff7a895b21a75c6d3923269f45bc84c81b0
Partially-implements: blueprint periodic-tasks-to-db-slave
Merge "Periodic task _heal_instance_info_cache can now use slave db"