Remove image_driver_list in zun.conf

We should use the default value for image_driver_list to support
both glance and docker images.

Change-Id: I123fce57cdbd7d154d70aa8d171f946f74dd4e2f

ansible/roles/zun/templates/zun.conf.j2

@@ -11,7 +11,6 @@ transport_url = rabbit://{% for host in groups['rabbitmq'] %}{{ rabbitmq_user }}
 
 state_path = /var/lib/zun
 container_driver = docker.driver.DockerDriver
-image_driver_list = glance
 db_type = sql
 
 [network]