Several roles are setting facts that are expected to be consumed by
roles following them; generally things like the path to installed
tools, etc.
Add a section to policy about this behaviour in general, noting it
should use the cachable flags to persist the values.
Add specific documentation notes in an "Output Variable" section for
roles currently implementing this behaviour.
Change-Id: I05657fec198176c7d7345e84293f4902c81fa30c
9e92459592