sync_power_states handles missing driver info

The ConductorManager._sync_power_states() periodic task will stop
checking power states if getting the power state of a node raises an
exception. Eg, this could happen if a node is not properly configured
(for which node.driver.power.validate() raises an exception).

With this change, sync_power_state catches the exceptions, logs them,
and continues checking other nodes.

Change-Id: I343f722de8f79a50fbaa9fcdd180decdc5a43f95
Closes-Bug: #1262912
Merge "sync_power_states handles missing driver info"