Add support of cinder volume driver to k8s CoreOS

Support of volume driver in coreos cluster create is missing though magnum document specifies that it is supported. cluster template can be created with --volume-driver cinder but cluster creation fails.

Reproduction steps:
1) Setup magnum with devstack
2) create keypair: testkey
3) create coreos image with name: CoreOS
4) create cluster template:
magnum cluster-template-create --name k8s-cluster-template-coreos-cinder \
                       --image-id CoreOS \
                       --keypair-id testkey \
                       --external-network-id public \
                       --dns-nameserver 8.8.8.8 \
                       --flavor-id ds1G \
                       --network-driver flannel \
                       --coe kubernetes --volume-driver cinder

5)magnum cluster-create --name k8s-coreos-cluster --cluster-template k8s-cluster-template-coreos-cinder --node-count 1 --timeout 120
The above command will fail specifying: volume_driver missing in template_def