sync for multiple volumes from same volumeset inefficient

[wallnerryan]

If you have a manifest like this

docker_app: cassandra.yml

flocker_hub:
    endpoint: https://data.flockerhub.clusterhq.com

volumes:
    - name: cassandra1
      snapshot: cassandra-snap-1
      volumeset: cassandra-vs
    - name: cassandra2
      snapshot: cassandra-snap-2
      volumeset: cassandra-vs
    - name: cassandra3
      snapshot: cassandra-snap-3
      volumeset: cassandra-vs

fli-docker will sync the volumeset for each snapshot or branch. This is because snapshots may be from different volumesets. But in the case that they are, it still syncs every time. Maybe have a mechanism to hold what volumesets have been synced during the current run and skip if previously synced