Allow some instance polling periodic tasks to hit db slave

If your deployment has a slave configured, poll_*instance* periodic
tasks will send their reads to the slave. If you have no slave
configured, there is no change to behavior.

Note for developers, I have targeted all the easy tasks. By this I
mean tasks that were already using the InstanceList object and the
get_all_by_filters or get_all_by_host methods. I will slaveify the
other tasks in seperate reviews since they also require conversion
to using objects.

Change-Id: I4bead5d350d633d726d4564b4bc1baec2e2977b7
Partially-implements: blueprint periodic-tasks-to-db-slave
Merge "Allow some instance polling periodic tasks to hit db slave"