fix(snmp): corrected behaviour

[Tbaile]

Ref:

• Disable SNMP by default nethsecurity#1028

[gsanchietti]

Add the uci set command to enable it on line https://github.com/NethServer/nethsecurity-docs/pull/152/files#diff-dafe45421035531cb27396c9b8c6996fb6d3545d6cf82e20a3173a7d1389855aR26

Also remember to add the /etc/init.d/<service> start

[Tbaile]

Changed the doc, however as per procd, probably disabling the running daemon is a bad idea:

root@Stable:~# /etc/init.d/snmpd status
active with no instances
root@Stable:~# uci set snmpd.general.enabled=1
root@Stable:~# uci commit
root@Stable:~# reload_config
root@Stable:~# /etc/init.d/snmpd status
running
root@Stable:~# uci set snmpd.general.enabled=0
root@Stable:~# uci commit
root@Stable:~# reload_config
root@Stable:~# /etc/init.d/snmpd status
active with no instances