[swss]: Flush flex counter DB in swss_serivce.j2 (#1029)

Author: marian-pritsak

files/build_templates/swss.service.j2

@@ -17,6 +17,7 @@ ExecStartPre=/bin/bash -c "while true; do if [ \"$(/usr/bin/docker exec database
 ExecStartPre=/usr/bin/docker exec database redis-cli -n 0 FLUSHDB
 ExecStartPre=/usr/bin/docker exec database redis-cli -n 1 FLUSHDB
 ExecStartPre=/usr/bin/docker exec database redis-cli -n 2 FLUSHDB
+ExecStartPre=/usr/bin/docker exec database redis-cli -n 5 FLUSHDB
 
 {% if sonic_asic_platform == 'mellanox' %}
 TimeoutStartSec=3min