Add support for multiple machineDeployments

[paulphys]

In the current state only the replicas field gets duplicated when adding a new machineDeployment.
Ideally it should allow multiple deployments:

      machineDeployments:
        - class: openstack-scs-1-30-v1
          failureDomain: nova
          name: md0
          replicas: 1
        - class: openstack-scs-1-31-v1
          failureDomain: nova
          name: md0
          replicas: 1