adding default value for NUM_NODES in bml

Change-Id: I9d203b5008e13295b6a5a49f4e9dc9d09c68e17d
diff --git a/jjb/metal3/job_bml_main_integration_tests.yml b/jjb/metal3/job_bml_main_integration_tests.yml
index 039d93c..0d80a3f 100644
--- a/jjb/metal3/job_bml_main_integration_tests.yml
+++ b/jjb/metal3/job_bml_main_integration_tests.yml
@@ -74,6 +74,10 @@
         name: TARGET_NODE_MEMORY
         default: 4096
         description: 'RAM size of the target host.'
+    - string:
+        name: NUM_NODES
+        default: 2
+        description: 'Number of baremetal servers.'
 
     properties:
     - build-discarder: