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/lib/nova_plugins/hypervisor-docker b/lib/nova_plugins/hypervisor-docker
index cdd9317..b5df19d 100644
--- a/lib/nova_plugins/hypervisor-docker
+++ b/lib/nova_plugins/hypervisor-docker
@@ -32,7 +32,7 @@
 DOCKER_REGISTRY_PORT=${DOCKER_REGISTRY_PORT:-5042}
 
 DOCKER_IMAGE=${DOCKER_IMAGE:-cirros:latest}
-DOCKER_IMAGE_NAME=cirros
+DOCKER_IMAGE_NAME=$DEFAULT_IMAGE_NAME
 DOCKER_REGISTRY_IMAGE=${DOCKER_REGISTRY_IMAGE:-registry:latest}
 DOCKER_REGISTRY_IMAGE_NAME=registry
 DOCKER_REPOSITORY_NAME=${SERVICE_HOST}:${DOCKER_REGISTRY_PORT}/${DOCKER_IMAGE_NAME}