Ignore Conflict exception when setting power state

The call to set_power_state() when registering an Ironic node can
fail if the Ironic conductor has grabbed a lock first and started
its work to assert power state. Since the conductor will power off
the node, we can safely ignore the exception.

Change-Id: I77eef1654b025e33b1b54d35f020d0b61d739e5a