if docker-volume-size of baymodel lessthan 3, bay cann't create

[Michael liu]

magnum is running on centos7,

magnum baymodel-create --name k8sbaymodel5 --image-id fedora-23-atomic-20160405 --keypair-id testkey --external-network-id public --dns-nameserver 8.8.8.8 --flavor-id m1.small --coe kubernetes --docker-volume-size 5

magnum bay-create --name k8sbay5 --baymodel k8sbaymodel5 --node-count 1

Execute the above command can get a completed bay,but when docker-volume-size is 1 or 2,the status of bay is FAILED.