Allow _sync_power_states periodic task to hit slave DB

This commit allows those who have slave_connection configured
to have reads from _sync_power_states hit that connection. This
change does not affect anyone who doesn't have a slave. The
behavior remains the same for them.

Implements: blueprint periodic-tasks-to-db-slave
Change-Id: Iaeb2d47a5064a649f427b18d5cbd2b6b97ce90d0
Merge "Allow _sync_power_states periodic task to hit slave DB"