Implement sync_power_state periodic task

Implement a periodic task in the ConductorManager which will check all
the existing, non-locked nodes and verify that their current power
states correspond to state stored in DB. If they differ, for now, log a
warning and update the DB using current power state.

This is a step towards keeping powered-off-nodes off.

Also, this removes the unused db.api.get_nodes method
and adds a new get_nodeinfo_list method.

Change-Id: I609fdbd7db6620de617b44f83c7ec35350f9178c
Blueprint: keep-powered-off-nodes-off
Merge "Implement sync_power_state periodic task"