Found the issue, it was partially me not reading the log carefully enough.
The log from my first post mentions directory "/var/run/cloudera-scm-agent/process/20-hdfs-NAMENODE-format/logs", in which I found stderr.log with the lines :
Running in non-interactive mode, and data appears to exist in Storage Directory /soft/dfs/nn. Not formatting. 16/01/12 11:15:37 INFO util.ExitUtil: Exiting with status 1
It turns out there were files remaning from a previous attempt, cleaning them solved the issue.