(#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 remote-tracking branch 'rob/ticket/master/19162-reboot' into ticket/master/19162-reboot

* rob/ticket/master/19162-reboot:
  (#19162) Finish applying, then reboot