When the run_once clause is present and set to a truthy value, the
instruction it is attached to will only be executed the first time it is
encountered.
* The `add_host` option can be used to add the current host to the
group being created. It avoids having to use a separate `add_host`
instruction in this case.
* The `parent` option can specify the parent group. It avoids having
to use a separate `add_child` instruction in this case.
The "block" instruction allows mulitple instructions to be grouped in
order to be executed based on a single condition, in a common loop, with
a local variable scope. In addition, it provides a way to recover from
errors.
