This adds optional notifications emitted when ironic changes a node's power
state or when ironic detects a change in a node's power state.
These notifications can be consumed by any external service listening to the
message bus to perform functions like tracking node power state changes over
time or automatically responding to anomalous power states.
The event_types of the new notifications are:
* baremetal.node.power_set.{start,end,error}
* baremetal.node.power_state_corrected.success
This also adds a new NodePayload class for notification payloads related
to nodes.
Change-Id: I82702e7f959d666bb02b59d1fc53ab50b519cb74
Closes-Bug: 1526408
ff32b51bbf