Add default for backupninja_backup_host for backward compatibility

Change-Id: I1d65b11a5db1e21eeddab27b3518f1204b303b65
Related-Prod: PROD-29006 (PROD:29006)
diff --git a/defaults/backupninja.yml b/defaults/backupninja.yml
index e89ed86..f827173 100644
--- a/defaults/backupninja.yml
+++ b/defaults/backupninja.yml
@@ -1,4 +1,5 @@
 parameters:
   _param:
     backupninja_engine: rsync
+    backupninja_backup_host: 127.0.0.1
     backupninja_public_key: no-key-provided