powerstrip-flocker does not pass through non-flocker volumes

[lukemarsden]

If a volume doesn't start with /flocker, then it doesn't get bind-mounted from the host at all.

This is surprising behaviour. Probably the correct behaviour is:

• If a volume starts with /flocker, mount the flocker-volume (moving it in place if necessary). This is the current behaviour.
• If a volume doesn't start with /flocker, just pass through the request unmodified to the docker daemon so that it mounts a local volume from the host.