It was reported in issue #6267 that the private key was leaked when
using the DEPLOY_HAPROXY_HOT_UPDATE=yes feature.
Indeed, the debugging code which sends commands to HAProxy was using
_debug even when passing the private key.
This patch fixes the issue by using _secure_debug when doing that.
Exporting SYNO_LOCAL_HOSTNAME=1 only works on the first deploy invocation. Subsequent deploys using the saved config fails because it saves $SYNO_HOSTNAME instead of $SYNO_LOCAL_HOSTNAME.