Change NCS_CONFIG_DIR to /nso/etc

This effectively moves the secrets (CDB encryption keys, SSH keys and SSL certificate) to the config directory. In docker fashion, the directory /nso/etc will be volume or a bind mounted in the container. Normally the users of NID/system would take backups of those files themselves (or not :)). This change makes it possible to use ncs-backup to backup (and restore) these files automatically, thus allowing someone to restore a backup even outside of NID/system.