backend.DemoteFromMC: don't fail for missing files

If the config file is missing when the DemoteFromMC() function is
called, it will raise a ProgrammerError. Instead of changing the
utils.CreateBackup() file which is called from multiple places, for now
we only change the DemoteFromMC() function to not call it if the file is
not existing (we rely on the master to prevent race conditions here).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>