Add a default value for the 'protocol' config option

Change-Id: I54229c63a41ed775cb036947197bca731fafa36f
diff --git a/config.yaml b/config.yaml
index 37c3b74..49b2cdb 100644
--- a/config.yaml
+++ b/config.yaml
@@ -24,7 +24,7 @@
       "use-multipath" option in the nova-compute charm.
   protocol:
     type: string
-    default:
+    default: iscsi
     description: |
       SAN protocol to use. Choose between iscsi or fc.
   volume-backend-name: