Ironic driver must wait for power state changes

The Ironic virt driver was previously returning from power_on,
power_off, and reboot calls immediately after making the request to
Ironic -- but before Ironic had necessarily acted upon it. This could
lead to Nova acting as though the state change was complete before it
actually was.

This patch introduces a new private method which polls Ironic until it
signals that the power state change is complete. With this, the
power_on, power_off, and reboot methods wait for the action to complete
before returning to ComputeManager.

Closes-bug: #1310135

Change-Id: I0bbdd8bc8807a0df43a420825f497842b387c4d5
Merge "Ironic driver must wait for power state changes"