Fix mongodb name in cron.json

There is a typo in cron.json making cron to fail
when mongodb is enabled.
Only affect master.

```MissingRequiredSource:
/var/lib/kolla/config_files/logrotate/mongdb.conf file is not found```

Change-Id: I79d41e94ee3361f2b4d831d17b1bb34a1964f580