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

Author: Hongbin Lu

lib/nova

@@ -823,7 +823,7 @@ function start_nova_compute {
         run_process n-cpu "$NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf" $LIBVIRT_GROUP
     elif [[ "$VIRT_DRIVER" = 'lxd' ]]; then
         run_process n-cpu "$NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf" $LXD_GROUP
-    elif [[ "$VIRT_DRIVER" = 'docker' ]]; then
+    elif [[ "$VIRT_DRIVER" = 'docker' || "$VIRT_DRIVER" = 'zun' ]]; then
         run_process n-cpu "$NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf" $DOCKER_GROUP
     elif [[ "$VIRT_DRIVER" = 'fake' ]]; then
         local i

stackrc

@@ -573,7 +573,7 @@ case "$VIRT_DRIVER" in
     lxd)
         LXD_GROUP=${LXD_GROUP:-"lxd"}
         ;;
-    docker)
+    docker|zun)
         DOCKER_GROUP=${DOCKER_GROUP:-"docker"}
         ;;
     fake)