(#19162) Finish applying, then reboot

Added apply with two options: immediately (default) and finished. When
`apply => finished`, it allows puppet to finish applying resources and then
reboot the computer at the end of the catalog run.

When someone selects `apply => finished` and `when => pending`, the behavior
will be allowed but will warn that this is an unsupported scenario. I don't
always understand the needs of the users so I didn't want to constrain this
option. If a sufficiently advanced user knows exactly what they are doing and
wants to set this behavior, it should be allowed.
Merge pull request #6 from joshcooper/ticket/master/reboot-when-finished

(#19162) Finish applying, then reboot