do not create cinder-volume init file on Ubuntu

If init file is created before the package is installed, the package
puts the real init file into /etc/init/cinder-volume.conf.dpkg.dist, so
file_line ends up adding a line into an empty file instead of the real
init file.
Merge pull request #739 from xarses/ceph-cinder-volume-init

do not create cinder-volume init file on Ubuntu