Add support for URL based volumes.

If the API looks ok, I will add it to the other API files.

thanks
--brendan
I think that it's two-fold.  I think that there is clearly a good role for here for side-car containers.  And having a standard library of side-car containers that do things like this.

However, I think there are two complicating factors:
   * side cars are always going to be harder to use correctly than api objects, since we can validate the semantics of the api objects.
   * since we don't have sequencing between members of a pod right now, it is tricky to use a sidecar for this style of initialization without blocking in the other container (which adds additional complexity)

For these reasons, I think that it is useful to add these kinds of utilities the API.

I think a compelling approach going forward is to perhaps have a volume type that wraps a user supplied container.  That would provide a bunch of the flexibility that you suggest, while still enabling a reasonable user interface and sequencing simplicity.