create cassandra user if backup.server defined

Previously cassandra user was created only if
cron job has been set up which is wrong as
manual backup script run failed being unable
to exec rsync.
Besides that server-side backup\restore scripts
now are owned by root user and not cassandra
user as only root user is specified in cron.allow
anyway.

PROD-20823

Change-Id: Iac71571cead0b447f29ca4bb6449697434fc33b0
1 file changed