[snmp] Check intfmgrd running before start (#16588)

Add pre start check to ensure intfmgrd is running.
The check will run for 20 seconds at most.

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

files/build_templates/snmp.service.j2

@@ -10,6 +10,7 @@ StartLimitIntervalSec=1200
 StartLimitBurst=3
 
 [Service]
+ExecStartPre=/bin/bash -c 'end=$((SECONDS+20));while [ $SECONDS -lt $end ];do if /usr/bin/pgrep intfmgrd >/dev/null;then break;else sleep 1;fi;done'
 ExecStartPre=/usr/local/bin/{{docker_container_name}}.sh start
 ExecStart=/usr/local/bin/{{docker_container_name}}.sh wait
 ExecStop=/usr/local/bin/{{docker_container_name}}.sh stop