.kitchen.dokken.yml: ensure verifier is found

The verifier is mounted on `/tmp/verifier` by default, which is masked
by Docker bind mounting the kitchen data before running init.  Thus
mount it instead to `/opt/verifier` so ServerSpec tests are actually
run.

https://github.com/someara/kitchen-dokken#tmpfs-on-tmp

Author: zakame

.kitchen.dokken.yml

@@ -12,6 +12,10 @@ transport:
 provisioner:
   name: dokken
 
+verifier:
+  root_path: '/opt/verifier'
+  sudo: false
+
 platforms:
 - name: centos-7
   driver: