Correct handling of volumes in workload config

Volumes are configured in tenant dict, and tied to servers via server dict. When
setting up server, find its volumes by display_name parameter.
Example:
CLOUDS:
    source:
        workloads:
            tenant:
                volumes:
                    - &volume-1
                        display_name: volume-1
                        size: 1
                servers:
                    -
                        name: server-1
                        volumes:
                            - <<: *volume-1