Add virt driver 'zun' to devstack

According to the feedback in the TC meeting [1], we renamed the Nova
virt driver from "docker" to "zun" [2] to avoid name collision
to nova-docker. This rename also help to clarify the difference
between these two drivers.

[1] http://eavesdrop.openstack.org/meetings/tc/2016/
    tc.2016-11-29-20.01.log.html
[2] https://review.openstack.org/#/c/414651/

Change-Id: I747080953ae4d1d35ed334831100413b6e4466c4
diff --git a/stackrc b/stackrc
index 7ce6c51..19f5b53 100644
--- a/stackrc
+++ b/stackrc
@@ -573,7 +573,7 @@
     lxd)
         LXD_GROUP=${LXD_GROUP:-"lxd"}
         ;;
-    docker)
+    docker|zun)
         DOCKER_GROUP=${DOCKER_GROUP:-"docker"}
         ;;
     fake)