Refactor out function to format instance policy

The new function can be used for group-level instance policies. Support for
roman numeral formatting has been dropped to make the code simpler. For
other info items it's already unsupported, and it's not exactly documented
anyway.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>