Add support for writing into files as part of the executor config
Add support for writing into files as part of the executor config

This allows us to set the value of sysctls in the config so we can test
different variants more easily.

Signed-off-by: Chris Redpath <ae54b883c23fde04daba73ac75796ae299bcb360@arm.com>

mdigiorgio/lisa,mdigiorgio/lisa,bjackman/lisa,arnoldlu/lisa,credp/lisa,ARM-software/lisa,bjackman/lisa,joelagnel/lisa,credp/lisa,ARM-software/lisa,arnoldlu/lisa,credp/lisa,ARM-software/lisa,credp/lisa,joelagnel/lisa,ARM-software/lisa