Can't download a file via os.system('scp (...)') whithin a python script launched via a init script - python-2.7

Question

I'm trying to download a remote file using the scp command called by os.system() in a Python script (scp.py) started as a service by a procd init script.
This script is running on OpenWrt 15.05.1 (which uses BusyBox to implement the shell environment). SCP itself is handled by dropbear SSH (https://matt.ucc.asn.au/dropbear/dropbear.html)
The code is quite simple :
import os
import logging
logging.basicConfig(level=logging.DEBUG, filename='/usr/local/www/log/scp_test.log', filemode = 'w')
rc = os.system('scp -i /root/.ssh/id_rsa root#vps500141.ovh.net:/root/simon/test /tmp/hello')
error_code, signal = rc >> 8, rc & 0xFF
logging.debug('error_code -> {}'.format(error_code))
logging.debug('signal -> {}'.format(signal))
When launched in the terminal using the command "python scp.py", the program's behave well. The download is successful and produces the following output :
root#S096C08:/usr/local/bin# python scp.py
test 100% 24 0.0KB/s 00:00
root#S096C08:/usr/local/bin# cat /usr/local/www/log/scp_test.log
DEBUG:root:error_code -> 0
DEBUG:root:signal -> 0
But this program fails to download anything when launched as a service using the command /etc/init.d/scp_test start
root#S096C08:/usr/local/bin# /etc/init.d/scp_test start
root#S096C08:/usr/local/bin# cat /usr/local/www/log/scp_test.log
DEBUG:root:error_code -> 1
DEBUG:root:signal -> 0
Furthermore, a quick investigation of the syslogs using the logread command shows this :
Wed May 15 10:24:58 2019 daemon.err python[3156]: Host 'vps500141.ovh.net' is not in the trusted hosts file.
Wed May 15 10:24:58 2019 daemon.err python[3156]: (ssh-rsa fingerprint md5 41:aa:2b:57:48:be:01:81:48:a3:d0:ac:b6:56:16:34)
Wed May 15 10:24:58 2019 daemon.err python[3156]: Do you want to continue connecting? (y/n)
Wed May 15 10:24:58 2019 daemon.err python[3156]: /usr/bin/dbclient: Connection to root#vps500141.ovh.net:22 exited: Didn't validate host key
vps500141.ovh.net already have an entry in /root/.ssh/known_hosts so this is quite surprising that it is not found when the script is running.
I've also tried to add the "-o StrictHostKeyChecking=no" option to the scp command but it doesn't seems to work either.
NB : this is the content of the init script /etc/init.d/scp_test
#!/bin/sh /etc/rc.common
# Copyright (C) 2015 CZ.NIC z.s.p.o. (http://www.nic.cz/)
START=99
STOP=0
USE_PROCD=1
SCRIPT="/usr/local/bin/scp.py"
start_service() {
procd_open_instance
procd_set_param user root
procd_set_param command python "$SCRIPT"
procd_set_param stderr 1
procd_close_instance
}

Answer 1

Problem resolved after moving /root/.ssh/known_hosts to /.ssh/known_hosts