Creates an empty init script override file if necessary

Without this patch, puppet runs were failing with the following error:

(/Stage[main]/Cinder::Volume::Rbd/File_line[set initscript env]) Could not
evaluate: No such file or directory - /etc/init/cinder-volume.override

Change-Id: Ibf42b4b09441c8a044f50af62246be31fe584e1b