(Maint) Deprecate the `prompt` parameter

The `prompt` parameter was intended to allow a reboot to be canceled
when running puppet interactively, e.g. for testing, demo, etc. However,
if you are running `puppet agent`, the prompt will cause a dialog box to
appear on the hidden service desktop, causing the agent to hang waiting
for user input.

This commit causes puppet to issue a deprecation warning only if prompt
is set to true. Leaving it unspecified or setting it to false, will do
nothing.