Feature: combine_output to control stdout/stderr

This enables an often-requested feature to separate stderr
from stdout within function responses. New flag combine_output is on
by default to match existing behaviour. When combine_output is set
to false it redirects stderr to the container logs rather than
combining it into the function response.

Tested with unit tests for default behaviour and new behaviour.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>