Serializing single tasks

Certain applications that run multiple copies of a service may not react well to all of those services being restarted at once. Typically, when upgrading this type of application, a serial play is used. However, if the application is of large enough scale, serializing the entire play may be wildly inefficient. A different approach can be used, which is to serialize just the sensitive task(s), often the handlers to restart services.

To serialize a specific handler task, we can make use of a built in variable, play_hosts. This variable holds the list of hosts that should be used for a given task, as part of the play. It is kept up to date with hosts that have failed or are unreachable. Using this variable, we can construct a loop to iterate over each host that could potentially run a handler task. Instead of using the item in the module arguments, we'll use the item in when conditional and a delegate_to directive, so that we can run the handler task if the host notified...