[master] fix config sflow enable failed

[tim-rj]

Signed-off-by: tim-rj sonic_rd@ruijie.com.cn

- What I did
fix sonic-net/sonic-buildimage#5803
- How I did it
make sure sflow unmask before enable and start
- How to verify it

config sflow enalbe

- Previous command output (if the output of a command-line utility has changed)

root@sonic:/home/admin# config sflow enable
Failed to enable unit: Unit file /etc/systemd/system/sflow.service is masked.
root@sonic:/home/admin# systemctl status sflow
● sflow.service
   Loaded: masked (Reason: Unit sflow.service is masked.)
   Active: inactive (dead)

- New command output (if the output of a command-line utility has changed)

root@sonic:/home/admin# config sflow enable
Removed /etc/systemd/system/sflow.service.
root@sonic:/home/admin# systemclt status sflow
bash: systemclt: command not found
root@sonic:/home/admin# systemctl status sflow
● sflow.service - sFlow container
   Loaded: loaded (/lib/systemd/system/sflow.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2020-11-04 10:46:01 UTC; 27s ago
  Process: 28203 ExecStartPre=/usr/bin/sflow.sh start (code=exited, status=0/SUCCESS)
 Main PID: 28281 (sflow.sh)
    Tasks: 15 (limit: 4461)
   Memory: 26.9M
   CGroup: /system.slice/sflow.service
           ├─28281 /bin/bash /usr/bin/sflow.sh wait
           └─28282 docker wait sflow

[tim-rj]

will close this one since it is cause by config_db.json.