Fix database initialization for db_migrator (sonic-net#3100)

What I did
db_migrator failed to initialize SonicDBConfig, and I fix this issue.

How I did it
If SonicDBConfig is already initialized, do not invoke initialize() again.

How to verify it
Run unit test, and verified on DUT.

scripts/db_migrator.py

@@ -8,9 +8,10 @@
 import re
 
 from sonic_py_common import device_info, logger
-from swsscommon.swsscommon import SonicV2Connector, ConfigDBConnector, SonicDBConfig
+from swsscommon.swsscommon import SonicV2Connector, ConfigDBConnector
 from minigraph import parse_xml
 from utilities_common.helper import update_config
+from utilities_common.general import load_db_config
 
 INIT_CFG_FILE = '/etc/sonic/init_cfg.json'
 MINIGRAPH_FILE = '/etc/sonic/minigraph.xml'
@@ -1241,10 +1242,7 @@ def main():
         socket_path = args.socket
         namespace = args.namespace
 
-        if args.namespace is not None:
-            SonicDBConfig.load_sonic_global_db_config(namespace=args.namespace)
-        else:
-            SonicDBConfig.initialize()
+        load_db_config()
 
         if socket_path:
             dbmgtr = DBMigrator(namespace, socket=socket_path)

tests/db_migrator_test.py

@@ -1,7 +1,8 @@
 import os
 import pytest
 import sys
-
+import argparse
+from unittest import mock
 from deepdiff import DeepDiff
 
 from swsscommon.swsscommon import SonicV2Connector
@@ -869,3 +870,18 @@ def test_golden_config_hostname(self):
         hostname = host.get('hostname', '')
         # hostname is from minigraph.xml
         assert hostname == 'SONiC-Dummy'
+
+class TestMain(object):
+    @classmethod
+    def setup_class(cls):
+        os.environ['UTILITIES_UNIT_TESTING'] = "2"
+
+    @classmethod
+    def teardown_class(cls):
+        os.environ['UTILITIES_UNIT_TESTING'] = "0"
+
+    @mock.patch('argparse.ArgumentParser.parse_args')
+    def test_init(self, mock_args):
+        mock_args.return_value=argparse.Namespace(namespace=None, operation='get_version', socket=None)
+        import db_migrator
+        db_migrator.main()