Fixed prepare.sh script to find the external net name

PROD: PROD-36810
Change-Id: Ib09cdd2d13f52919477dd36cd6ae272185538ed5
diff --git a/k8s/rally-files/openstack-mos-scn-i100-static-users.json b/k8s/rally-files/openstack-mos-scn-i100-static-users.json
index b4f1c75..0eecdbc 100644
--- a/k8s/rally-files/openstack-mos-scn-i100-static-users.json
+++ b/k8s/rally-files/openstack-mos-scn-i100-static-users.json
@@ -2,7 +2,7 @@
 {% set image_name = image_name or "cvp.cirros.51" %}
 {% set availability_zone = "nova" %}
 {% set concurrency = 10 %}
-{% set volume_size =  %}
+{% set volume_size = 1 %}
 {% set times = 100 %}
 {% set users = 5 %}
 {% set tenants = 4 %}