[db_migrator] Migrate DNS configuratuion (sonic-net#2893)

What I did
sonic-net/sonic-buildimage#14549
This PR has introduced DNS configuration to CONFIG_DB, we need to migrate original CONFIG_DB to support DNS configuration during upgrading SONiC.

How I did it
If there's no DNS_NAMESERVER table in CONFIG_DB, read DNS_NAMESERVER table from minigraph and update CONFIG_DB.

How to verify it
Run unit test for db migrator.

scripts/db_migrator.py

@@ -1089,6 +1089,7 @@ def version_4_0_5(self):
         This is the latest version for master branch
         """
         log.log_info('Handling version_4_0_5')
+
         return None
 
     def get_version(self):