[pmon] Fix chassis_db_init exit not being expected (#9858)

- Why I did it
Error log was shown on switches during boot
pmon#supervisord 2021-12-22 04:27:16,709 INFO exited: chassis_db_init (exit status 0; not expected)

- How I did it
Add exit code zero as an expected exit code and also disable autorestart.

- How to verify it
Boot the switch and ensure the above log line does not appear.

dockers/docker-platform-monitor/docker-pmon.supervisord.conf.j2

@@ -46,7 +46,8 @@ dependent_startup_wait_for=rsyslogd:running
 command=/usr/local/bin/chassis_db_init
 priority=3
 autostart=false
-autorestart=unexpected
+autorestart=false
+exitcodes=0
 stdout_logfile=syslog
 stderr_logfile=syslog
 startsecs=10