Set DEFAULT_IMAGE_NAME for Docker hypervisor

This allows the tempest configuration to set the right
image for booting docker containers with Nova.

Since glance uploads are not yet integrated in devstack,
IMAGE_URLS remains empty.

Change-Id: I5df153cd1d5e1411bb3c11816122ce280148e129
diff --git a/stackrc b/stackrc
index 7eed60c..d754f3b 100644
--- a/stackrc
+++ b/stackrc
@@ -280,6 +280,9 @@
     openvz)
         DEFAULT_IMAGE_NAME=${DEFAULT_IMAGE_NAME:-ubuntu-12.04-x86_64}
         IMAGE_URLS=${IMAGE_URLS:-"http://download.openvz.org/template/precreated/ubuntu-12.04-x86_64.tar.gz"};;
+    docker)
+        DEFAULT_IMAGE_NAME=${DEFAULT_IMAGE_NAME:-cirros}
+        IMAGE_URLS=${IMAGE_URLS:-};;
     libvirt)
         case "$LIBVIRT_TYPE" in
             lxc) # the cirros root disk in the uec tarball is empty, so it will not work for lxc