Use --tenant-id, not --tenant_id

Change-Id: I0e3d65d5b69ac82cbf7ee6ffc41ead369af8c126
diff --git a/lib/savanna b/lib/savanna
index 6794e36..bb4dfe6 100644
--- a/lib/savanna
+++ b/lib/savanna
@@ -56,7 +56,7 @@
     SAVANNA_USER=$(keystone user-create \
         --name=savanna \
         --pass="$SERVICE_PASSWORD" \
-        --tenant_id $SERVICE_TENANT \
+        --tenant-id $SERVICE_TENANT \
         --email=savanna@example.com \
         | grep " id " | get_field 2)
     keystone user-role-add \