Fix to improve hostname handling (#12064)

* Fix to improve hostname handling
If config_db.json is missing hostname entry, hostname-config.sh ends
up deleting existing entry too and hostname changes to default 'localhost'

* default hostname to 'sonic` if missing in config file

files/image_config/hostname/hostname-config.sh

@@ -3,6 +3,11 @@
 CURRENT_HOSTNAME=`hostname`
 HOSTNAME=`sonic-cfggen -d -v DEVICE_METADATA[\'localhost\'][\'hostname\']`
 
+if [ -z "$HOSTNAME" ] ; then
+       echo "Missing hostname in the config file, setting to default 'sonic'"
+       HOSTNAME='sonic'
+fi
+
 echo $HOSTNAME > /etc/hostname
 hostname -F /etc/hostname