Start RestAPI container when sonic boots (sonic-net#4140)

* Start RestAPI container when sonic boots

Author: sumukhatv

files/build_templates/restapi.service.j2

@@ -0,0 +1,16 @@
+[Unit]
+Description=RestAPI container
+Requires=updategraph.service
+After=updategraph.service
+Before=ntp-config.service
+
+[Service]
+User={{ sonicadmin_user }}
+ExecStartPre=/usr/bin/{{docker_container_name}}.sh start
+ExecStart=/usr/bin/{{docker_container_name}}.sh wait
+ExecStop=/usr/bin/{{docker_container_name}}.sh stop
+Restart=always
+RestartSec=30
+
+[Install]
+WantedBy=multi-user.target