Truncate the container and pod ID files instead of throwing an error.
The main motivation is to prevent redundant work when starting systemd
units. Throwing an error when the file already exists is not preventing
races or file corruptions, so let's leave that to the user which in
almost all cases are generated (and tested) systemd units.
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>