Fix parameters name

Fix param names for creating VNs to match the names used in
opencontrail salt module.

Change-Id: I71f53beafb6d074378697c841a79e26e894dc92d
diff --git a/opencontrail/client.sls b/opencontrail/client.sls
index ff67a3e..671402f 100644
--- a/opencontrail/client.sls
+++ b/opencontrail/client.sls
@@ -243,8 +243,8 @@
   - name: {{ vn.name }}
   - conf:
 {%- if vn.ip_prefix is defined and vn.ip_prefix_len is defined %}
-      ip: {{ vn.ip_prefix }}
-      prefix: {{ vn.ip_prefix_len }}
+      ip_prefix: {{ vn.ip_prefix }}
+      ip_prefix_len: {{ vn.ip_prefix_len }}
 {%- endif %}
 
 {%- if vn.asn is defined and vn.route_target is defined %}