Fix: initialize SonicDBConfig differently for single or multi_asic (sonic-net#1409)

**- What I did**
Fixes sonic-net#6708

This bug is exposed by sonic-net/sonic-utilities#1392. Previously the `config` command will call `SonicDBConfig.load_sonic_global_db_config()` even on a single ASIC platform, and it will silently failed. After exposed, it will fail with error syslog message:
```
admin@sonic:~$ sudo config save
Existing files will be overwritten, continue? [y/N]: ^CAborted!

Feb  8 03:23:48.729434 sonic ERR sniffer: :- initializeGlobalConfig: Sonic database config global file doesn't exist at /var/run/redis/sonic-db/database_global.json
```
**- How to verify it**
Tested on DUT

Author: qiluo-msft

config/main.py

@@ -931,7 +931,11 @@ def config(ctx):
         platform.add_command(mlnx.mlnx)
 
     # Load the global config file database_global.json once.
-    SonicDBConfig.load_sonic_global_db_config()
+    num_asic = multi_asic.get_num_asics()
+    if num_asic > 1:
+        SonicDBConfig.load_sonic_global_db_config()
+    else:
+        SonicDBConfig.initialize()
 
     if os.geteuid() != 0:
         exit("Root privileges are required for this operation")