[config] Check golden config exist early if flag is set (#3169) (#3504)

### What I did
Fix  #3164
Check Golden Config earlier before service is down.
#### How I did it
Move the check at the begining
#### How to verify it
Unit test

Co-authored-by: jingwenxie <jingwenxie@microsoft.com>

config/main.py

@@ -1746,6 +1746,15 @@ def load_minigraph(db, no_service_restart, traffic_shift_away, override_config,
  argv_str = ' '.join(['config', *sys.argv[1:]])
  log.log_notice(f"'load_minigraph' executing with command: {argv_str}")
 
+ # check if golden_config exists if override flag is set
+ if override_config:
+ if golden_config_path is None:
+ golden_config_path = DEFAULT_GOLDEN_CONFIG_DB_FILE
+ if not os.path.isfile(golden_config_path):
+ click.secho("Cannot find '{}'!".format(golden_config_path),
+ fg='magenta')
+ raise click.Abort()
+
  #Stop services before config push
  if not no_service_restart:
  log.log_notice("'load_minigraph' stopping services...")
@@ -1817,12 +1826,6 @@ def load_minigraph(db, no_service_restart, traffic_shift_away, override_config,
 
  # Load golden_config_db.json
  if override_config:
- if golden_config_path is None:
- golden_config_path = DEFAULT_GOLDEN_CONFIG_DB_FILE
- if not os.path.isfile(golden_config_path):
- click.secho("Cannot find '{}'!".format(golden_config_path),
- fg='magenta')
- raise click.Abort()
  override_config_by(golden_config_path)
 
  # Invoke platform script if available before starting the services